It definitely isn't difficult to understand a British person. You will find, however, that there are more dialects within England than there are in the States. People from every little nook and cranny have their own accents. Therefore, you will find it quite easy to understand most people you speak to, but some will pose a little more difficulty. Don't be afraid to revert to the "stupid American" excuse, and tell them that you don't understand.

While help can't be offered when it comes to understanding accents, there are lots of slang words that would be beneficial for you to know.