White Hound wrote a lengthy (almost 60,000 words) Britpicker's Guide, with sections on food, wildlife, sports, clothes, profanity, et cet. There are subsections like “Things Britons don't eat at breakfast (but Americans apparently do).” Her site also has a list of British cultural references in the books, which I think that every American writer of HP fic should check out. On Reddit, you might also get some use from What film tropes just don't work in a U.K. setting?, which has a couple thousand comments.
If you want to maintain British spelling, then you can change the native language of your word processor by following the instructions below. If you use a program that isn't listed here, then let me know and I'll add it to the list.
I haven't yet found any free online tools for Britpicking your spelling, except for websites like Reverso, which will correct American spellings if you ask it to do so but limits you to 450 characters at a time. I really recommend using a word processor (if your word processor of choice isn't in the list, then let me know and I'll add instructions).
It's worth noting that nouns which refer to multiple people are treated as plurals: you wouldn't say “my family is” or “the government is,” like an American, but rather “my family are” or “the government are.”
American writers should save a link to the Wikipedia article, “Glossary of American terms not widely used in the United Kingdom,” as well as pages A-L and M-Z of “List of words having different meanings in American and British English.”
Other useful articles:
The British are not so religious as Americans, and Evangelicals and Fundamentalists are much rarer. You are especially unlikely to see anything to do with exorcisms, especially when Tom Riddle lived at Wool's Orphanage (neither the Church of England nor the Catholic Church were keen on exorcisms at that time), and modern exorcisms are generally more sedate than Hollywood films might've led you to believe. They also include visits by a psychiatrist to make sure that the supposed victim of possession isn't just in need of medical care.
Speaking of Wool's Orphanage: Institutions like Wool's were phased out in the 1950s and no longer exist. Sad as it is, Harry Potter cannot grow up in an orphanage.
As for other miseries that we are unfortunately unable to heap upon poor Harry Potter, it makes no sense for the Dursleys to deny medical care to Harry on account of the doctor costing too much. Despite the best efforts of the Conservative Party, the National Health Service yet lives, and it was doing fine enough in the 1980s and 1990s, too. Check out Useful Notes / NHS for a basic rundown on how healthcare works when you don't live in America. Glasses in particular are subsidized for children (and some others), so the Dursleys should only complain about the cost of glasses if they're lying.
“I'm still undecided about Tom Riddle. Trevor Doom, now…that's different.”
“You're going to have to change that, you know.”
“Change what?”
“The name. Trevor Doom, LL.D. It's missing letters. Tom Marvolo Riddle?”
“Okay, Trevor Iam Doom, LL.D.”
“Iam?! That's not a name!”
“Of course it is. Short for Iambus. Perfectly valid name, Iam. Come to think of it maybe he used it. Iam comma Lord Voldemort — that's a name. Lord Voldemort on its own is just a title — and title as in land at that. There'd have to be some place for him to be lord of, a marshy little cul-de-sac called Voldemort somewhere. Place with a lot of swamp gas, ignis fatuus. Probably in Jersey.”
ForestUUID, “ Potter Who and the Wossname's Thingummy ”
Important disclaimer: Behind the British peerage there stands a thousand years of bullshit, and exceptions to nearly every rule. Consider the Earl of Selkirk, a title governed by rules of inheritance that are even more bullshit than usual.
There are five titles of British peerage: duke, marquess, earl, viscount, and baron. Older titles, or titles which were created earlier in history, are “more senior” than other titles, which mostly matters when you need a tiebreaker in your aristocratic dick-measuring contest. The most senior title is the “premier” of that title, e.g. premier duke, premier baron.
The titles of “hereditary peers” are inheritable, and the titles of “life peers” (or “working peers”) are not. Since 1958, life peers have been created on recommendation by the Main Honours Committee, itself composed of eight sub-committees: Arts and Media, Community, Economy, Education, Health, Science and Technology, Sport, and the State. The results are typically in batches on regular special occasions, such as the monarch's birthday. Since 2000, there have also been “people's peers,” recommended by the House of Lords Appointments Commission; their titles are uninheritable as well. Until 1999, a register of hereditary peerages was kept on the Roll of the Lords Spiritual and Temporal.
As originally practiced, proxy voting was conducted by sending someone of lesser rank in one's place. By the early 1600s, proxies were typically selected from among other members of Parliament, though in 1626 members were banned from casting votes for more than two proxies at a time. Proxy voting has been banned in the House of Lords since 1868.
When a title is formed, it is said that a peer has been created. The children of the peer, or title-holder, are the peer's issue. Where it is believed that there is an heir somewhere but they cannot be found or their status cannot be proven, then the title falls dormant. When it is believed that there is no heir, the title becomes extinct. Obviously, it is possible for a title to be considered dormant but really be extinct. Titles that have been renounced or relinquished are said to have been disclaimed. The terms under which a title may be inherited are collectively referred to as that title's remainder.
Laws of inheritance are specific to a title, and generally refer to three kinds of heir: (1) heirs male of the body, who descend directly from the previous holder of the title; (2) heirs male whatsoever, who need not claim a direct line of descent; (3) heirs whatsoever. Inheritance may be limited to legitimate or “legally begotten” heirs, but “legitimacy” can often be obtained very late, even through performing a marriage while the mother is in labor. With regard to these requirements, one may speak of heirs-male, heirs-general, etc. Adopted children cannot inherit titles in the Muggle world, but things may be different for witches and wizards.
The eldest eligible child of the peer is the heir apparent ; if the peer has no children, then the next claimant in line is called the heir presumptive. If the peer dies without eligible children then the heir presumptive inherits, unless the peer's widow says that she is (or may be) pregnant, in which case inheritance is held off until the matter can be resolved.
There were once also “baronies by tenure,” which were held simply by possession of land and a castle, but these became increasingly uncommon over the generations until they were entirely abolished in 1861.
If a title may be held only by male heirs but the previous peer, upon his death, had only daughters, and at least two of these daughters have eligible sons, then the title may “fall into abeyance,” which is to say that it is not held by anybody. The title remains in abeyance between the “co-heirs” until such a time that they come to an agreement regarding the inheritance of that title, which must not have been in abeyance for more than one hundred years; or, until such a time that only one possible heir remains. In the former case, the co-heirs must then petition the Crown so that the title may be “called out of abeyance.”
Dukedoms were originally created by the fastening of a ceremonial sword to a belt or girdle, but since 1615 this practice was replaced by the conferral of letters patent under the Great Seal. In speech, dukes and duchesses are addressed as “Your Grace.”
Let's say that Rodolphus is the Duke of Wereham, the Earl of Ingoldisthorpe, and the Viscount of Stoke Ferry, that he and Bellatrix have three children — Soteira, Lesath, and Sagitta — and that the inheritance laws in Norfolk pay no regard to sex or gender. Rodolphus himself would be Rodolphus Lestrange, 10th Duke of Wereham, and he might be called “Duke Wereham” or, among that special liminal zone of acquaintances close enough for familiarity but not close enough for first names, just “Wereham.” Bellatrix would probably sign papers as “Bellatrix Wereham,” using her husband's title rather than his surname. Together, they would be the Duke and Duchess of Wereham.
Soteira, using her father's second-best title, would be Lady Soteira Ingoldisthorpe (and heir-apparent), and her brother would be Lord Lesath Lestrange. If the laws permitted only male heirs, then her brother would be Lord Ingoldisthorpe. Either way, the youngest Lestrange child would be Lady Sagitta Lestrange, even if she married outside of the aristocracy. If, however, Sagitta married Patrick Cornfoot, 11th Earl of Kilmany, then she would adopt his title and become Lady Kilmany, or Countess of Kilmany.
Later, Soteira marries Cadogan Mockridge, a fourth-born child with little chance of inheriting a title, and they have a child, August. Because August is, so to speak, Rodolphus Lestrange's grand-heir (he is due to become the Duke of Wereham someday, presuming that Soteira lives long enough to become Duchess ahead of him), he will be able to use his grandfather's third-best title as Lord August.
Generally speaking, unmarried children are referred to by both their Christian name and surname (i.e. Lord Lesath Lestrange, Lady Sagitta Lestrange), but the spouse of a peer uses only their last name (i.e. if Lesath married Joan Sharp, 4th Baroness of Wemyss, and was not due to inherit any titles from his parents, then he would adopt her title and become “the Lord Wemyss”).
Husbands typically do not get titles from their spouses (Queen Elizabeth's husband was Prince Philip, not King Philip), so if Soteira instead married Harry Potter, the son of a wealthy but untitled burgher, then he would remain Mr. Potter (in a more egalitarian society, this might not be the case), though their firstborn child, Hedwigus, would be the Viscount of Stoke Ferry just like her alternate-universe half-brother August. If both spouses have a title, then they will both use the superior title (assuming that the spouse with the inferior title is able to use their spouse's title).
As Jo Beverley says, “Unusual situations do tend to get complicated.”
Marquessates are created by letters patent under the Great Seal. The rank is not especially popular; there are nearly as many extant dukedoms as there are marquessates. In speech, marquesses and marchionesses are addressed as “My Lord.”
Suzanne Bones (not to be confused with Susan Bones) is the 5th Marchioness of Clavering. She may also be called Lady Clavering, or simply “Clavering,” and her husband, Art Bones, is the Marquess of Clavering, or Lord Clavering. As the peer's spouse, he would sign papers as “Art Clavering.”
Earldoms are typically life peerages or hereditary and inheritable only by direct male heirs, but some Scottish earldoms can be inherited by women or through the female line. Like dukes, they were originally created by ceremonial sword shindiggery but since 1615 have been created by letters patent. In speech, earls and countesses (there is no other feminine form) are addressed as “Lord [Whatever]” and “Lady [Whatever].”
Peadar Dukelow, 13th Earl of Riverstick, will be the Earl of Riverstick, or Lord Riverstick, or simply Riverstick, as described above. His husband, Francis Merrythought, conveniently has no title, so we can easily say that he would sign papers as “Francis Riverstick.” Their son, Hawkin, does not have an ordinary courtesy title.
Viscountcies were first created in 1440 but were unpopular until the 1600s, so it's plausible that there are few to zero wixen viscounts. They are created by letters patent. In speech, viscounts and viscountesses are addressed as “Lord [Whatever]” and “Lady [Whatever].”
Baronies were originally created as life peers via a summons by Royal Writ to attend the King's Counsel or Parliament. There was no assumption of inheritance, but in practice a baron's heir would usually receive their own summons, and on and on, until the practice solidified and baronies were hereditary by default. Since the 1400s, baronies have typically been created by letters patent. In speech, barons and baronesses are addressed as “Lord [Whatever]” and “Lady [Whatever],” and never described as “Baron [Whomever].”
Courtesy titles in summary: If a duke, marquess, or earl has multiple titles, then their heir apparent may use one their lesser titles. Younger sons of a duke or marquess have the courtesy title of “Lord [first name] [last name],” and younger sons of an earl, as well as all children of a viscount or baron, have the courtesy title of “The Honorable [first name] [last name].” Daughters of a duke, marquess, or earl have the courtesy title of “Lady [first name] [last name].” Whatever the details, courtesy titles are usually taken up at the age of majority (i.e. Soteira Lestrange does not become Soteira Ingoldisthorpe until she is seventeen) and do not grant a seat in the House of Lords.
Below the barons are the baronets and knights, who are not peers, and whose chief distinction from each other is that baronetcies are inheritable and knighthoods are not. Baronets and knights are addressed as “Sir [first name] [last name],” e.g. “Sir Arcturus Black,” and their wives as “Lady [last name],” e.g. Lady Black. Female baronets and knights are addressed as “Dame [first name] [last name],” e.g. “Dame Poppy Pomfrey.”
As Ermithecow points out, titles may (rarely) be connected to names, e.g. Lucius Malfoy, 17th Earl Malfoy. “It would make sense that wizarding peers aren't linked to land because the muggles would know who they were in that case,” though it would also make sense for Muggle and magical jurisdictions to overlap. Another word from Ermithecow: “it is absolutely not the done thing that teachers or other authority figures refer to children by their titles.” Soteira could ask the teachers to call her “Ingoldisthorpe” rather than “Lestrange” (once she's old enough) but not “Lady Ingoldisthorpe” or anything similar.
For terms of address in writing, you should consult The English Manner, because the details vary depending on the formality of the message.