I am going to suggest you watch a few episodes of 'Only Fools and Horses'. It's an 80s sitcom set in Peckham, South London. The show is practically considered a national treasure in the UK and most people who grew up in the UK are very familiar with it.

The reason I suggest it is that it showcases a very particular dialect of British English that many people struggle with (even native English speakers) if they aren't familiar with it. The show was so popular in fact that many modern colloquial British English phrases/words can trace their appearance to this show, not because the show invented them, but because the show exposed so many people to this dialect.

Some examples of words/phrases popularised by OFAH: dipstick, wally, cushty, lovely jubbly.

See http://www.bbc.co.uk/comedy/onlyfools/lingo/ for more.

If you want to master modern British English, check it out. It's also hilariously funny.