Wheever you go people don't much like strangers who breeze in with a superior or sneering attitude, no matter where they come from. They might be from a city a few miles away, or a visitor from the other side of the world, it makes no difference. Except that people are a bit more tolerant of that stuff from someone from the other side of the world.

Barring that, no probs. Nobody blames visiting Americans for Bush.
On today's Stop the War march in London there were plenty of Americans with American Flags and very welcome too.

And most pubs can let you have chilly beer in bottles, and noone will look strange at you for asking for it, because lots of natives drink it that way. No accounting for tastes.