I'm the founder of Protected content - short term renting in social network. You can post your flat for free when you are away and earn extra income by renting only to the people you trust - your friends of friends, members of the internations, alumni of top business schools, etc.
As a guest, you can save up to 80% from hotel prices. It's ideal if you are just moving to London (or any other city) till you find a long term accommodation.
Let me know if you have any questions or if I can help with specific searches.