Sadly, the survey's only in central London!
I just spent a week in North Norfolk: my HTC One (4G-capable) phone on O2, my wife's 3G Sony on Orange (yes, effectively EE), and my daughter's 3G HTC on EE.
About half the time, I couldn't get a signal at all on O2. On the occasions when I could, GPRS was the best I could hope for: no good for anything but occasional mail polls. I did once see 3G, but it lasted about a second, and did find HSDPA once, in one medium-sized town. A truly lousy performance.
On the other hand, the two EE phones had a voice signal almost all the time, and often could get HSDPA when I couldn't even see a signal.
This reflects my experiences in other (out of London) areas too. And this nicely demonstrates how different the mobile operators' strategies are, for coverage. There's population coverage, and there's area coverage. Try to use Waze, for example (a GPS app that requires online data access), and you find out the difference pretty quickly: O2 is not exactly the ideal network, even if they do manage good(ish) coverage in town and on major routes. I've an EE MiFi-style data hotspot (which I forgot to take to Norfolk, natch*). When I'm travelling off the main routes, I use that to provide wi-fi data to my O2-limited mobile, if I want to navigate. Or NavFree, which doesn't need online access. It shouldn't have to be this way.
Suffice to say: when the current contract's up, early next year, I'm sacking O2, and porting my number to EE. And I can't imagine even looking back afterwards.
(* Yes, I know I could have used one of the other phones as a wi-fi hotspot, but they're both on low-data tariffs.)