Well,I couldn’t help to argue that why you’re so anxious to learn local accent in London?Even a lot of British people cannot get the typical London accent very well sometimes.If you wanna learn typical British English including British accent,New Concept English will do the trick.And as for the pronunciations,imitation is always important.Kids could imitate everything naturally with no problem but adults don’t.The efficient way that linguists suggest is to record your own words and match with the native one that you’ve followed and practice in this way from time and time again.It’s definitely a real tough job but in all of sudden you will figure out that you can handle it.Believe me,follow the right way and all it takes is to practice.Hope you will make it.