When I let my friend play one of my Xbox games online, he specifically wanted to use the mic just to talk shit. It's not that Americans are trying to rag on Brits because they are Brits, that's just how we play video games in America. We talk shit in an attempt to throw people off of their game. Especially with FPS's, it's a competition.
I'd be willing to bet MMO's don't have nearly as much trash talk from Americans. In fact, when I played an MMO, a couple of Australians decided to try and rag on me because I didn't know the specifics of how to play.
It's one of the reasons why I don't play with a mic, simply because I don't like trash talk. But when I do for things like reaslism mode or hardcore mode, I am ready to talk shit back.