It's Ofcom's job to keep those cheeky mobile networks in line, and it's released maps of the nation's 3G coverage to show how they're living up to their 3G commitments.
You couldn't use it to find out if you're house is covered -- unless you live in the Highlands, where you're pretty much screwed from every direction -- but it's interesting to see how the different networks stack up.
Keep in mind that this only shows 3G coverage, so you could still make 2G voice and data calls from inside the blank spots. And it doesn't show the quality of the coverage, and you might get better data speeds from different networks in the same area. Also, there are some simplifications -- Ofcom assumed that all trees and buildings are 10m tall, for example, and that you're calling from outdoors.
Ofcom says it's now going to work on getting coverage to the places that aren't getting any 3G love, such as rural areas.