Put simply, London is the United Kingdom in its entirety. London spreads itself throughout the island like butter across a hot crumpet. London folk are all neighbours with Her Majesty the Queen and often share afternoon tea with her. Linguists believe this is the cause of the phenomenon where all Londoners speak exactly like the aforementioned queen.
Random Englishmen: I am from the north of England.
Random Americanmen: Oh? My friend Julie lives in London, do you know her?
Random Englishmen: Why yes I believe I do, I’ll be sure to tell her we met.
Random Americanmen: Thank you, safe travel back to London!