The one I hate is the purely American, "How are you today?" when you go into a shop. I've no objections to Americans saying it as it is part of their culture. However, Brits know better and they know that they should be coming out with Good Morning/Afternoon.
My stock answer to it is, "Same as I was yesterday. Why do you ask?" and sometimes they're even aware enough for it to make the point. The last time this happened was in the Rohan shop near Covent Garden and the bloke cracked up at my dazzling wit leaving me thinking "uh?". He was, of course, American. At least he didn't get offended.