I brought my iPhone with me who I was on Verizon with. I just popped my old SIM card out after getting a new one from giffgaff and works fine.
I think originally I had a prepaid sim from tesco then went on a sim only plan from virgin media and now have switched to giffgaff.
It's wonderful in the U.K. You don't have to be on a contract and it's much cheaper
I don't know how much data you use but I think I have one or two gigs plus 500 mins? Incoming calls don't count and unlimited text for ten pounds a month. I never go over my data or minutes.