A lot of Forum members, me included use a Three Data sim - there are 2 variants available, a 12GB/12 month or 24GB/24 months sim.
These are available on Amazon or from MyMemory website
www.mymemory.co.ok.
There are often deals on these sims, but generally you pay £32 for the 12GB and £45 for the 24 month one.
Hope that helps.