How to talk in a british accent?
Study up on some monty python, as those guys are all British. Record your speech, words, etc, while trying to imitate the british accent to make sure that you're talking the right way. After a while of practice, you can talk in a British accent well enough to fool some people.
simply, make british friends and talk with them, or ask them to teach or whatever, find your own way, it's easy... I can talk in an American accent, thanks to mummy :)
Stay with your British couisins for two weeks and you will pick something up.