that's because if you got 3G activated on your phone, but a bad coverage, your mobile phone will search a better network 24/7 (=huge power consumption)!
Yes i know!
As far as 3g goes, i live in brighton, see this link. http://www.webmap.o2.co.uk/map.asp
Good coverage (for VIDEO CALLS ie 3G) so the phone is NOT having to strain to hold on to a connection and the battery life with 3g on is still bad.