You should be able to get quotes for the accommodation on the uni website, and halls usually have bills included, so you wouldn't have to factor in anything above for that. It is possible to survive on about £20 a week for food, even in London, but I'd say that £50 a week was more realistic. It also depends on whether you can walk to your lecture building or whether you will need to pay for transport, as that's a huge addition to living costs.