O2 are out of 3GS at the moment.
Basically if you start your contract through them you will end up with "Hello sir, no if you want help with that you need to call o2" followed by o2 saying "Hello sir, no if you want help with that you need to call Mobiles.co.uk"
For this reason, and for the sake of £60. Id rather just order it direct from o2.