Um... also, although I realise I'm in danger of going off topic, and don't have the time to a quick bit of research to check I'm not talking tat, but...
Most of you probably know that many European languages have both singular and plural (or formal singular) words for 'you'. So in French, it's tu and vous. In Middle English, thee was the equivalent of tu. But modern English, for some reason, has only retained you for both French words tu and vous.
I am pretty sure that in SOME, old and obsolete cases, 'you was' would have been correct. It would have been the correct verb conjugation for formal singular.
I've no idea how London English evolved, but I think you'll find some regional speech patterns retain old forms of speech/grammar, rather than misusing new ones. So you kinda have to think back to front about it, if that makes sense. I think 'you was' is a hangover from days long gone in many cases.