Have found Three to be superb.
I can take or leave the customer service, frankly they are all about the same, but the mobile & data services on Three are superb for the money, and barring a few very dodgy locations I have never had a problem with signal (a common quibble that used to be levied at Three). Works fine in some stupidly remote parts of the country, oddly enough though not at my parents which is on top of a mountain, but in a recessed dip and an ancient building with uber thick stone walls, right outside the house no problems though.
All you can eat data, unlimited calls and texts, I think Three are hard to beat.