I think one of our favourites would have to be saying "good" when someone in the States ask "how're ya doing today?". We're so used to saying "fine" over here, it takes some getting used to saying "good, thanks". "Fine" often gets a strange look
My problem is that a few Britishisms creep inot my speech from time to time. Some examples are: Bloody (and sometimes bloody hell), daft, mate, bonk (sorry), knickers, etc...