If you hang around a British person long enough, you are practically guaranteed to hear him make a derisive quip about Americans.
In all likelihood, he won’t even notice that he’s done it. I have been at many dinner parties at which it has been baldly stated, without embarrassment or regret, that Americans are “all fat” or “all stupid,” or . . . well, you can pretty much choose your epithet at random, and I have watched with irritation as the line was met with general agreement. It is a peculiar thing that reflexive anti-Americanism is tolerated in Britain and beyond to a degree that no other rudeness is. On occasion, I have tried to point this out, puckishly inquiring as to whether the speaker would make the same charge with a different nationality. “Nigerians are all stupid,” perhaps? I think not.