A friend of mine uses Giffgaff. If you order the free SIM card from
this referral page, you will receive an additional £5 free credit when you top up by £10 (giving you a total balance of £15) and my friend will also receive £5 of free credit for introducing you.
Given that you need £12 to get
unlimited data for one month (as well as 250 minutes and unlimited SMS), paying £10 will give you this £12 bundle and an additional £3 to spend on other things like international calls and SMS.