Ladbrokes, mainly. Main reason being that their online service is linked with their stores. Some like Coral distance themselves completely with their online stuff so if you have a problem you're on the phone on a 0845 number instead of just sorting it out on the spot.
Have a shop 'round. I'm sure many sites would offer more than a £10 freebet. I think Paddy Power had £50 last week. Matched betting is pretty big amongst students though I never bothered with it. Moneysavingexpert might have the best deals posted somewhere. Good luck!