As previously mentioned, regardless of what network you are thinking of signing up to, that is if you are thinking about taking out a contract, try them for a good while with a pay as you go phone.
Some networks are better than others in different parts of the country, if Im going to go for a sumary then it would be:
Vodafone Good overall coverage in most places, crap for data
O2 - again good overall coverage a little more expensive, data a bit better than Vodafone
T Mobile/Orange/EE Good signal in most places, expensive, data good
Three: good Signal in most places however this can struggle a bit in older ( ie thick stone walled ) buildings, and outside in dense woodland/ bottom steep valleys - but this is countered by an excellent value for money, and a usually excellent 3G signal and data speed that can put 4G to shame