http://www.bbc.co.uk/worldservice/learningenglish/multimedia/pron/unit1/start.shtml

As usual, BBC offers a variety of very nice exercises to practice British English. This is a good website to find out which are the most important differences between American and British accents. The vowels component is very comprehensive in this site.

http://international.ouc.bc.ca/pronunciation/

In here you can find tongue twisters, listening exercises on a specific phoneme. I personally suggest the consonants component