Buy £5 airtime Get £5 free at Easymobile.com
An extra £5 airtime if you port your number to easymobile. I think they still offer £5 airtime for Friend referrals. In priciple, up to £40 in credit & cashback for a £5 spend.
Quidco cashback: £12 for a sim pack and purchase of initial credit (within 60 days of successfully registering the sim pack) - register your sim immediately. If you only register but dont buy any credit, Quidco cashback will be only £5