On many trips to Britain I've occasionally encountered rudeness to Americans. In my experience they are the same angry leftist boors who act the same way about President Bush and conservatives in America. The thing to do about them is ignore the poor benighted souls, certainly not to let them ruin a day or a trip.
Yes, just ignore the leftists, don't get into an argument with them because they will probably burst a blood vessel or something. But above all, please remember that the prejudices of a few miserable leftist bigots are not representative of the British people's views.