Hey mate.
I'm also from England, and I faced a similar problem. Basically, my juvenile Corn lives in my room, which is always at a constant of roughly 74 degrees. This is obviously the cool temperature, and the warm side stays at about 84-85 degrees. So, you could make sure that the radiator is at a constant temperature. As long as the warm side stays constant, and 70 degrees or just under is as low as it gets, then you shouldn't face too many problems. If it gets even colder, then you may wish to move them to a room where you can keep the temperature constant.
However, there's another way of keeping the temps constant without having to face uncomfortable temperatures in the house. If your snakes vivs are large enough (like at least 30" long by 18" by 18") then you could have either two UTHs or one UTH and a heat lamp. One heat source could be kept at 80 - 85 and the other kept at 70 - 75. If you can afford this then this would be the best option. I have to do this with my Royal Python, otherwise it would get far too cold for her.
Hope that helps
David