I'm in North Wales, in a small village around 10 miles south of Chester, on the outskirts of Wrexham. Anyway, coverage is fairly good, but that's probably a result of not just an A-road a mile or so away, but the fact it goes around a small hill... so they have one north of the hill, one south of the hill and one over to the west (near me) sited near a cemetery on top of another hill, to cover where the road goes west around the first hill [between the first pair of masts]. The first two were numbered LL0001 and LL0002 and the last one LL0078, so I assume they added it some time later when they found some gaps in coverage.
I checked on the Ofcom web site and found there are 3 cell masts for Three (because of that hill). There is one mast for T-Mobile, 1 for Vodafone and 1 for O2, and a few for Orange. You can check which masts are near you at Ofcom's SiteFinder and for higher speed internet access look for 'UMTS' or '2100 MHz' (frequency).
Hope that helps with finding which services are available where you are - you should also get coverage information for your postcode (and places you expect to go with your iPad) from the different network web sites.
Yes, using PAYG does mean you can chop and change but once you've found a service you like, you will likely find it cheaper to go for a contract. (However, the Three 'One Plan' might be worth asking about in a Three shop, since they seem to still be offering internet access without stingy limits.)
Using the PAYG service, you may find your usage is actually quite low - I know there have been threads where people indicate they have used only 500 MB in a month. I was using my (second hand) Nokia N80 the other day and some pages I looked at were 1 MB alone. I downloaded a few items and those ran to 5 or 10 MB each, but all the time I was using home wi-fi connection (fortunately) so was not worried or counting the pennies (or pounds).