I love this:How to speak like a true Londoner: https://www.youtube.com/watch?v=E2xQFKwmLoQ
What always surprises me is that many people only think there is 1 American accent and 1 British accent. From what I have read there are at least 8 different American English Dialects, and probably many local dialects and accents In Britain, people say, there are dozens of regional dialects and accents.
There are also many Native English accents that are neither American nor British.
@Chris, oh, I love this guy's videos! There's another one, about Cockney rhyming slang; I found it when searching for the explanation of the "deep-sea diver" expression (rhymes with "fiver" -> £5). I don't know how accurate it is but I think it's fascinating, really!