The more interesting thing is how most of that slang comes from Jamaican slang. Except for wasteman. Hell they even say “wah gwan”.
London's black population IIRC isn't as diverse as say NYC.
It's primarily the Jamaicans and the Africans (chiefly Nigerians and Ghanians). Yes, you have other Carribeans nations represented but those from what I encountered were the main ones. And yea, a lot of Jamaican influenced slang in Black London slang.
I was saying 'Man dem' back in Nigeria years ago and that was imported from the British Nigerians.