Honestly I have the same paranoid feelings, and I've never to my knowledge been called out on my Americanisms, though I'm pretty sure my fic is littered with them.

At first I was okay with it so long as my dialogue wasn't overtly American, but apparently that is not okay at all. Pretty much the result has been my not writing a lot of Harry Potter fic that I would have liked to simply because I feel like it's going to become hated even if beta-ed or something, and my reluctance to step into Merlin fic writing.