It's difficult to say - you can buy a 3G repeater, but they're usually expensive and restricted to commercial applications. Most mobile phone companies these days share masts in rural areas - it's just too expensive to install your own infrastructure for the limited population. Looking at the coverage maps is usually the best way to see what chance you stand of getting good reception but maybe you have friends or colleagues or even neighbours who have a different network provider to you? Another issue is whether any coverage would be 3G or 2G. The older network operators, such as Vodafone, have upgraded their urban networks to 3G but left their rural networks as 2G, because the economics were simply not there. Only '3', as far as I'm aware, has an all-3G network in the UK, because 3G uses different frequencies to 2G and '3' wasn't around when the 2G licences were awarded. As a result, when they rolled out their network, they were forced to go 3G all the way. So, if you can receive '3' you know you'll be getting a 3G connection whereas for Vodafone, say, you'll probably only be getting 2G - which will give your GPRS or EDGE - much lower datarates and not really suitable for web browsing.
If you already have a WiFi only iPad a better option is probably going for a SIM-free MiFi unit. I have the '3' Huawei E585, which is available SIM-free widely. You could then try it with a series of low-cost PAYG SIM cards and see which works the best. I find this MiFi gets really good reception - more signal bars than my iPad and both are on '3'.
Tim
