The Student Room Group
What happens is that... you keep trying to get accomodation and in the meantime, you can move into some temporary accomodation - my university has a list of reccomended accomodation which included hostels as a suggestion of accomodation to use for a short time if you're in dire need of accomodation and your contract isn't due to start for a week or two. I'd contact your university's housing services and ask them what to do and what sort of help they offer. But beware that other than economic help, there's very little to do. London rent is just that expensive, so if that's going to be an economic problem that no student loan, scholarship, grants and/or Bank of Mum and Dad can't fix, then you might want to look into studying in a cheaper place.