3G connectivity is often patchy and unpredictable, as evidenced by the sight of a whirligig commuter frantically waving his smartphone around to latch onto a better signal. To help gauge it better, the Beeb has rolled out a map of 3G coverage across the country. Type in your postcode, and it'll give you a coloured map showing pockets of 3G, 2G, and no coverage at all. The map can also be filtered by service provider.
The data were compiled by some 44,600 Android phone users, who each downloaded an app that monitored their mobile coverage for a month-long survey. No correction in bias toward location / mobile provider was made, so the results aren't particularly scientific. The results have also been augmented by data from OpenSignalsMap.
With the gradual increase of wifi across the capital (coming soon to the Tube, buses, and pubs), your 3G data connection might not be all that important, but at the very least the map will give you some ammunition when you march into your local Orange store and complain about a lack of coverage in your manor.