You'll find spareroom a good place to start, as well as Gumtree (I've heard there are scams there sometimes, but I've used it 3 times and never had any problems)
You can use estate agents, but they're expensive and usually morons.
If you're moving to London, bring a lot of cash and be prepared to pay a deposit on the first visit to the place. There's probably not much room for negotiation, although where I live now is the first place I tried to get a discount and I succeeded. Anywhere else in the UK you should be able to do a deal and get 10% or so off the asking price. Be prepared to walk away.
This is my first post by the way, hello to everyone! :-)