Signal
It really will be a case of trying them to see which is the best.
I worked for O2 for 10 years and we had a great coverage (land mass wise)
But as with any network, some are better than others in certain areas.
Eg some areas we were the best and other areas, others were better.
Ofcom should have a coverage checker on their site, as do the networks on their webpages.
The crux of the issue is that a mobile, by definition is mobile, so you pay for access to service where available. Varied by things like the demand on the network in the area you're in (how many people are using the service at the same time as you in the same area). The distance you are from the mast (which generally you're not going to know) weather at the time, and any obstructions (trees, hills, buildings etc).
No network can guarantee 100 percent coverage 100 percent of the time.