Actually there usually is a London allowance on top of the normal stipend for studentships at London institutions. I'm hoping to get one myself, and while it won't be a huge amount (somewhere from 15,500-17,500 depending on where the funding comes from) I think it should be enough for me to manage on for 3 or 4 years without going into debt. I guess it depends where you will be living/studying, as housing and travel costs will be the main thing.