I'm not sure what O2 UK you think you used, but I've used O2 in the middle of national parks like Kielder Water and the Northumberland National Park and had pretty good signal. It's one reason I keep a phone on O2, as the old state incumbent network they have extensive cover in areas others can't cover (like national parks where planning permission is a problem).
I also have a phone with Three UK (the newest network in the UK) and while it cannot quite compete in really rural areas, it still covers a hell of a lot more than just the cities (the entire network is also at least 3G DC-HSDPA).
I cannot remember the last time I had a dropped call on either network.