I once met a telephone engineer in the Lakes, which would be about three years ago and he told me that Vodaphone has the most reliable signal. I've no real idea if this is true or not?
The nature of the terrain makes the provision of a reliable mobile signal very expensive and it is unlikely to be realised.
Vodaphone have a contract to use O2's network, so in most cases they will have similar coverage - and incidentally O2 was given an award for the broadest coverage, so you've got that going for you if you want a chance at a solid connection - though I expect mileage will vary.