3 may have a good plan, but trust me from someone living in England, its signal is awful, worse than AT&T's.
the best one I'd say is O2 for signal, followed by Orange.
OFCOM's maps say the exact opposite. Three's coverage and speeds are the best in the UK by a country mile, and O2's the worst. My personal experience travelling the UK and trying microSIMS from all the networks backs this up. Their coverage has been so impressive, I've switched my iPhone over to them from O2. I now have both devices connected to Three for half what I was paying O2 for the iPhone alone, and better coverage to boot.