When originally set up, the Vodafone and O2 networks used GSM-900 (900 MHz) whilst Orange and T-Mobile used GSM-1800 (1800 MHz). I think that, nowadays, Vodafone and O2 also have some GSM-1800 nodes.
The signals at 900MHz, being at a lower frequency, will go round corners and penetrate into buildings better than those at 1800MHz. But coverage will still depend heavily on the number and location of base stations.
http://en.wikipedia.org/wiki/Cellular_network
http://en.wikipedia.org/wiki/GSM_frequency_ranges
Links to coverage maps / calculators:
http://maps.vodafone.co.uk/coveragev...b/default.aspx
http://webmap.o2.co.uk/?Search=Search
http://coverage.orange.co.uk/uk/UKCoverageSearch.htm
http://www.t-mobile.co.uk/services/c...eck/?ref=quick
http://www.three.co.uk/personal/help...age_/index.omp