i moved from o2 to t-mobile for business and have had a lot better service, from the network and customer service.
if you want a referal let me know, might be able to get you a month free
Three are better than O2 round here and that's saying something. They'll probably be shit if I go into the sticks, and I was in Sheffield the other month and in a residential area I had no three signal at all inside which was a surprise. But Yorkshire is backwards with these things anyway.
Also, I'd agree that T-Mobile are generally great for signal and speed.