During my visits to Warwick, I didn't have any reception issues with my iPhone on O2. Most of the networks (O2, Vodafone, T-Mobile, Three) have masts on campus (on top of the library I think), so signal should be good for them. I've heard Orange is a bit more hit and miss though, and it depends a lot on what part of the campus you're in. For example, in the Rootes Social Building, I only had a couple of bars 2G (no 3G) with Orange. Some of the accommodation blocks are quite a distance from the main campus though, so reception in one location may not be the same somewhere else!