Have you considered that it isn't that they don't want to support you financially when studying in London, but they are unable to?
You've said yourself that they don't have much spare money - maybe they don't want you to go to London and scrape by, when you could live comfortably elsewhere?
Besides, why should your parents support you financially? It's nice that they can, but they don't HAVE to. So to be honest, stop being so selfish and ungrateful.