Due to a most unfortunate upbringing, I speak "
Pittsburgh English". I've spent many hours in accent improvement classes trying to shake its horrors. I have one big hurdle left (my inability to pronounce words like "dawn" and "talk" correctly). It's embarrassing when even non-native English speakers ask me "Where the hell are you from?"
So I sympathize. I've I've learned to adapt to Britishisms in books and they don't really bother me, unless they're used inconsistently.
One thing that drives me crazy is when British characters in books don't speak British. E.g. if they say "sidewalk" instead of "pavement." Gah.