Why are so many British people so prejudiced against Americans? Its not our fault....?

I'm certainly not saying that ALL British people are prejudiced against people in the US, but it seems like there is a lot of it. For example, JK Rowling said in an interview that she was worried about working with an American movie company (the WB). This seems to be the general attitude, and I really don't like it. I know that there is clearly a history with Britain and the US, and many people disagree with the actions of the American President, but that doesn't mean that anyone should be prejudiced against us as a whole. I don't know, I was just wondering if anybody had any insightful comments about this?