AND DONT go with O2,the customer service is appalling.BE WARNED
Add to that the network is deteriorating by the day. Just moved to three after a decade on o2, and whilst Three has a weaker signal in most places (though o2's signal isn't always 3G whereas 3 is), Three certainly has better blanket coverage.
Been all over the North in the past few days, work phone is on Voda, then I have my Three and o2 phones, and Voda seems the best blanket coverage, though 3G signal is intermittent, Three seems the second best though now that they have ceased roaming with Orange in most places, there is the odd blackspot, and o2 is great for GPRS/EDGE (sometimes), but 3g is pathetic, intermittent at best, but an increasing number of blackspots are appearing.
Using o2 mobile internet during peak hours can be a right pain in the backside, works phone does not have internet allowed on it so can't tell, Three is consistently ultra-fast (presumably because o2 is congested to the point of overload, and Three has much less users, also the fact that Three is 3g/HSDPA across 99.9% of it's coverage whereas o2 seems to be less than 50 overall, despite the claims they make).