On the subject of writing 'British' English, try watching the King Arthur Roundtable discussion a few times to get the gist. Also any interview with a British actor will help and there's a few of them on the Internet in printed format. There's arguably few identifiers that point out "American" English as opposed to "British" English. Just don't call a bathroom that unless it's got an actual bath, a lorry is NOT a semi, and pants are your undies, and football is soccer. Generally, you want to keep the way they speak fairly plain without Brit or Americanisms (after all, what they spoke way back bears no relation whatsoever to what Brits speak now and what they speak now varies wildly from county to county - ask any Londoner what some Suffolk farmer said and he'll shrug and say "I dunno.") Other than that feel free to consult darklyscarlett (hey, she volunteered first!). Also, I lived in England for a number of years during my formative years (the wild teenage ones mostly) and my nieces live there now - they love to answer questions. I like the suggestion of Canterbury for the Mansion - both a lovely location and appropriate, plus many available pics on the Web for inspiration. Or you could situate it in the general area of Hadrian's Wall (there are many lovely towns up there, but a lot of the countryside is rather forbidding and remote - a popular touristy thing is to do a walking tour where you hike from town to town and stay the night at various hotels and inns). Though I know a lot about the care and cleaning of an automatic pistol and how to fire one, and the care, cleaning and skills of firing various rifles - I don't know diddly about guns makes and models. While putting in the details like Laurell K. Hamilton does is nice, it isn't necessary. I read a large number of mysteries with various types of heroes and the majority of them don't put gun details in. They leave it vague. Unless it has something to do with your plot (like Laurell's 'how DO you hide a decent gun under a fancy sexy cocktail dress anyhow?') what do we need to know for? You're doing a fantastic job! Keep it up!
PeeK