Ooh! I get to geek out about my degree subject! :woohoo:
What you're talking about is a sociolect that linguists call Multicultural London English.
A sociolect is a dialect spoken by people of a particular social class. It's useful to distinguish that from your typical dialects because whether or not you speak that way is determined less by geography and more by social class. For example, a kid with middle class parents might speak very differently to a kid with working class parents, despite growing up in the same area.
The influences that cause somebody to speak with one accent versus another in this case are varied, but examples include what makes an individual fit in with their peers, how they've been taught to speak by authority figures and unconscious influence from speakers in their community.
A good example of a sociolect in early 20th century London would be Cockney. To massively oversimplify, people from more working class backgrounds would speak more like Ronnie Cray, and people from more middle and upper class backgrounds would speak more like the Queen. That sort of dynamic had been going on since at least the Norman conquest, and probably much longer.
What happened in the late 20th Century is that people started immigrating to the UK, especially London, from the Caribbean and bringing their dialects with them. Because these people tended to be less well off, they would mostly be sharing spaces with more working class people (again, to massively oversimplify).
These dialects therefore started to both influence and be influenced by the local working class dialects more so than the more middle and upper class ones. The result of this is that working class white kids who'd never been to the Caribbean in their lives nonetheless grew up alongside kids whose parents were from there, and so were naturally using elements speech from dialects from that region.
How many of those elements they incorporated into their own speech would be down to the individual, but enough elements transferred across that the default working class sociolect in certain areas of London became noticeably different enough to previous generations that it's now been given its own name and wikipedia page: https://en.wikipedia.org/wiki/Multic...London_English
I love this stuff, me.
:geek: