Why Are All Vampires British?

The UK's ambassador in Washington, Nigel Sheinwald, doesn't know, either, but thinks it's a good thing:

It seems an immutable law of Hollywood that the finest baddies in American film are always played by Brits. I'm not sure exactly why - there is nothing inherently menacing, I think, about an English accent. But whatever the reason, I have spotted a cinematic trend here that builds on this fine tradition: British vampires.