Gucci - welcome to the crazy world of aviation. To answer your questions:
1) All PPL training is now done for a JAR licence. JAR stands for Joint Aviation Requirements and is the result of the harmonisation of the various aviation bodies in Europe. There's not much difference between the old UK CAA PPL and the JAR PPL - except an extra 5 hours training.
2) You can do a JAR PPL in America so you don't need to convert anything when you get back. You will probably need a few hours though to get up to speed with UK airspace, radio procedures etc.
Good luck.