Three (also known as 3 or Hutchison) have 7GB for £25 on Pay as you Go on their USB modem stick option (i.e. the non-contract option). three dot co dot uk is their website.

Other pay as you go networks offer less data as a maximum. And some do not allow you to top up another discounted pack within a month even if you exhausted the first pack.

Second choice would be T-Mobile.

PS Giffgaff (for calls) and Tesco would be worth looking into too - I gather Tesco has a cheap data deal.