All four digital networks give fairly wide coverage populated areas of Great Britain, though one2one coverage is less widespread than its competitors'.
One of the best ways to see what coverage is really like is to use Autoroute, which is sold as a route-planning program - but you do need the overlay files. Note that the latest Microsoft versions do not support overlays. The versions that support them are no longer available for sale.
Orange provide more-or-less fortnightly updates of the coverage overlays on the Orange web site. They also have an interactive map and postcode look-up.
One2one have released Autoroute overlays from time to time, available from their web site.
Cellnet have overlays, but they are not on their web site for some reason. Contact me for details of where to get the files. The latest I have seen is dates January 1998, and gives coverage for December 1997, but tells you that it is January 1997 coverage! They do have up-to-date maps you can look at on their web site, though these are coloured to make the coverage look better than it is.
Vodafone once made their overlays available on their web site but stopped doing this, with the pathetic (and probably untrue) excuse that Autoroute wasn't y2k compliant. They also have coverage maps on the site, but they are nothing like as good as the overlays were.
There will probably be areas that GSM never reaches, as usable inland range is unlikely to exceed 5 or 6 miles, as opposed to the 30 mile range of TACS phones in good conditions. There is an argument that says that a mass market will make isolated base stations economic, but we'll have to see what happens.
TACS claims well over 98% coverage on both networks (but for carphones, not handportables, which are what most people use), but it isn't hard to find large areas with no service. particularly in hilly or remote places, and the networks are closing TACS down.
Why coverage is always expressed in population is a mystery to me, as mobile phones are most needed where there are no fixed phones handy!
There are networks of low-orbit satellites planned, providing mobile phone service where GSM isn't available. There are technical and political snags to overcome, though, and don't expect it to be cheap. The old satellite phone services are very pricey indeed (several pounds per minute for poor call quality), and Iridium (the only one of the new sort currently in operation) has had technical and financial problems, and is going out of business.
Comments, suggestions and amendments welcomed!
cellular. v1.58 12 Oct 2000