I’m working on a novel set in London (where I’ve lived for a while) and wondering how British I can make my own writing and the dialog before it’ll be considered too much.
Not very. I can’t give you specific guidelines as to how much you can use dialect or phrasing, but there’s a common refrain in publishing, “This is too British,” and you obviously are looking to avoid that. My suggestion: read other books set in London, Scotland, Ireland, and Wales to truly see how much is too much.
And ask the readers; when you have to stop to translate a phrase or conversation, it’s hard to restart again.