All network providers are good if you have a signal. And living in a city or town there's no reason why you shouldn't have a good signal. However, if you live in a remote area, affected by hills and valleys(Wales is renowned for black spot areas) you may find that signal strength is poor to non existent. This is the acid test for providers to provide! If you don't get a good steady signal then ring up and complain. They will(or should) send a field engineer to do a 'drive by' signal test. It may mean that coverage in your area simply needs antenna optimisation so you get a better and stable reception. It may also mean your area is a known blackspot area and the network provider will need to install a 'streetworks site'(pole with 360°antenna for immediate area)
Only EE, 3, Vodafone and O2 have mobile phone cell sites, the others use either of these.
My opinion is EE offer the best UK coverage and the best customer care. They may not be the cheapest but I'm a believer in you get what you pay for and have found them excellent since I switched from Vodafone. The worst I found have been Vodafone. Once market leaders they are probably doing worst out of the four now and offer terrible customer care, that's when you eventually manage to get through to them to complain!