Which British accent?

There's the "standard" called RP (for Received Pronounciation) also known as the BBC English, there's the rich people's accent (yeah, rich people in Britain have their own accent) known as Posh English, then there is a poor/working class Londoner accent called Cockney Accent (which outside Britain you often hear in TV series taking place in working class London neighbourhoods or when showing poor people in 19th century London), then there are a number of regional ones just in England (though those are harder to explicitly recognize if you're a foreigner, even if for example you can tell that somebody from Manchester has an accent different from somebody from Essex), then there are the other nations of Britain (Scotland, Wales, Northern-Ireland) which themselves have one or more accents each (I know for sure Scotland has more than one accent since I can notice the difference).

Mind you, I only know this because I lived there for over a decade.