I currently live in London. Most flatshare options can be found here. There are local papers with flatshare ads as well but by the time the paper has been published you find most of them have been taken.
You would probably only need a week staying in a hostel/hotel to view rooms before finding somewhere you can move into. Planning ahead is not really necessary and would probably cost you more. You may find it takes longer than you think to find somewhere you're happy with though. There are loads of cheap hotels and hostels for the transitional week you can find on google. If you're stuck, start with Hostelling International and work either way depending on your price.
If you have tried to get in to halls but didn't make it there are a few private halls springing up (e.g. Unite) but I think they tend to be quite expensive compared to private flatshares.
If you do go for a private flatshare try to share with other students as this means you won't have to pay council tax (local government tax for local services).
Hope that helps.