The problem is that in a city like Bath, it could be good in some areas and poor in others. I'm going through the same thing (just not in Bath) - my contract is finally up with TMobile and I want an Iphone, so I'm shopping around. My solution (a little long winded, but it worked) was to order free sim cards from the contenders (so one from O2 and one from Vodafone) and then borrow an unlocked phone from a friend. So now I know for sure what works where! It was most important to me that it work in my house, in my garden, at work, and in the town nearby where we most often go out, but the maps on the Vodafone site weren't precise enough....
ETA: slightly off topic, but to people who want an Iphone but are with a provider that doesn't advertise their availability, when your contract is up, tell them you're leaving because you want an Iphone. As it turns out, more providers than just O2, Vodafone and Orange have them, but they only apparently offer them up to current customers who say they're leaving!!!
