I think a lot depends on whether you want to go penny-pinching for the cheapest deal out there, or whether you'd prefer to go with the best carrier out there and concentrate on finding the best deal they have.
In my case I don't really care about a few bucks here or there - I simply want to enjoy the best speeds and coverage on offer. In the UK this is extremely simple: go with EE.
The competition is lagging behind and even price-wise you'll often get a better deal with EE than with other carriers, something that came as a nice surprise during my last trip to the UK. Either way, I wouldn't bother with anyone else as EE stands a class above the rest.