After the olympics it should be easier. Lots of flatsharing websites, spareroom.co.uk is one of the best, you can do lots of research before you come. If you stick to it you should find a flatshare in a few days' searching, but you have to be in London to do it because you must meet your new flatmates face to face. Couchsurf or arrange to stay on a friend's floor for your search. Then be systematic; book in three viewings every night, trudge round, judge a place first on the people, second on the location (walk to/from the tube), third the amenities. No point in an ensuite bathroom if your housemates are tossers and you don't feel safe walking back from the tube at night. It will be cheaper than renting a flat, but it won't be cheap anywhere in London.