Change the Panning permission rules first!
Surely half the problem is getting planning permission for the masts because the public don't want these eyesores 'in their back yard'? This certainly applies to the road I live in, the old folk just don't like 'em. I know its a royal pain in the ass in some areas, a change in law should be looked at by Mr C before pushing operators into looking at making drastic changes to their infrastructure. If one operator has a mast and coverage in one rural area then relaxing planning permissions so that masts that are approved can be shared by multiple service providers is worth looking at first?
I don't know much about the way it all works but I know for a fact that we had a mast up in my road for one operator which annoyed the hell out of the 'residence association', it went though but then a subsequent mast was applied for by another Telco and that one was rejected,, a pain in the ass because that was for the operator I was using at the time.