I've noticed that far to many writers use vast amounts of American slang for characters who are from the UK. This is one of my major pet peeves about fan fiction. If they're British, have them speak like their British!
I, myself, am not from the British Isles and therefor do not know how to 'talk the talk' so to speak. During my research for the perfect British Sland Term for the word "steal" I came across the following site:
http://www.effingpot.com/slang.shtmlIt is perfect for having authentic British characters. It's the little things like those words that make elevate otherwise good fics into greatness.