Don't let it keep you from writing fanfiction! That's awful! As I've said, much of the British-fandom fanfiction I've read by American writers has been wonderful; I may notice things like 'cell phone', and they may bother me a bit, but they're not enough to keep me from enjoying a good story. If you're really worried about Americanisms, just get a British person to look over your story before you post. Why 'even if betaed'?