The general view of about five years ago, when I had some links to the industry, were that o2 had about the best coverage throughout the uk.
But on Dartmoor there are, and probably always will be, very lage areas uncovered by any mobile signal because, well, there ain't no masts nor enough population to warrant them. (And people like me would probably object if they did start planting them everywhere!)
Satphones are falling in price all the time - a quick google shows you can get one, with a package, for under 550 american dollars (a search on a uk site shows them at around £700) and while it is much bigger than a mobile, possibly smaller than three mobiles?
Personally, and you may well put these words on my gravestone, part of Dartmoor's appeal is that it's still dangerous at times. Most of my life is cossetted and protected by legislation and cotton wool and to get out on the moor where there are real risks without signing and guarding and hazard tape is a refreshing change - and I'm not even an adrenaline junkie!
But... I do carry a phone because, well, I'm not an idiot
