I have learned over the years , that these things are VERY dependent on your location(s) , I'd rank the Networks (in terms of Coverage tat is useful to me)

O2 (=1)
Three (=1)
T-mobile(=1)
Vodafone (2)
Orange ( no personal experience , but at work I am always hearing people moan about it ! ) (3)

The three I have put joint first all seem to have decent coverage pretty much every time I call upon them. but others may find their services woeful
With PAYG sims so freely available now , its worth picking up one from any networks you are interested in , sticking it in an old handset and carrying it around for a few days , just to see what the signal meter does before commiting to anyone.