That English people all have awful teeth. I'm sure this has been mentioned many times but I haven't bothered to read through.
Probably the one most annoying stereotypes I see (sometimes here) is that we are all ignorant, lazy, fat people who are ready to shoot people on the slightest whim. Sure, that's an exaggeration, but I've seen Brits write things similar to this. It also confuses me when Brits say we have no culture, and then they will say that our culture is invading theirs. Obviously if our country's culture is "invading Britain," it's obviously because people are buying it.
It's also annoying that we can never win when it comes to the rest of the world. Everything we do is wrong. And if we should have done something and didn't, everybody is mad at us. In the eyes of the world, we can't do anything right. And it annoys me that China was scolding us for our human rights problems?
During my five years living here in the States I've had Americans assume:
* I consume tea by the gallon
* I have met (or even know!) at least one member of the Royal Family
* That I know John/James/Thomas/Fred the English chap they once met on a holiday in Florida
* That I think London is the best city in the world