I don't know who the cheapest is. I am with O2 and have been for many years as they have the best network coverage in my area. They have always been helpful when I've had to phone them, their call centre is in the UK.
we're with 02, have been for years and see no reason to change, good coverage, good service. The roaming charges are clearly laid out. What does catch OH out sometimes are the GPS location charges when he's outside the UK, but you can switch that facility off.