Giffgaff (O2 network) is great, you can buy a 1 month bundle of your choice, you can choose, or not, to have that bundle reoccur. So you can treat it like a contract if you want to, or you can cancel it at will if you do not.
Also as above, you can buy credit, and use that as you wish also.
I was with giffgaff for around 8years and only changed to smarty (3 network) recently due to O2 signal issues currently in my area. But they are a similar outfit.
