Write in your most natural voice; and with your canonical spellings. The only British-isms that I have trouble getting my head around quickly are the over-casual dropping of place names and neighborhoods in London. But we Americans grow up reading plenty of Dickens and Orwell and Shakespeare and whatnot, so just like a British accent we can handle it as long as it's consistent.

In general, you will do quite well to have even your American audience think, "Here's an well-written Brit."