In practice, it's they who find you. Just change the address on your linked in profile to "London" and see the emails coming in. If they don't come, your CV is probably not a good fit to the current needs of available contracts (due not enough experience in technologies that are currently hot).
Look for smaller companies and individual recruiters first. If they get you an interview in under 48 hours for a non shonky outfit and are willing to discuss your terms rather than enforcing theirs then you're probably ok. The big ones are shit. Requires phoning around for a couple of days.