O2's coverage is the thinnest on the ground -- not good news for iPhone users. In 2008, Ofcom warned O2 that if it didn't pull its finger out and improve its 3G coverage, it would take back its 3G licence early.
http://www.ofcom.org.uk/radiocomms/...and/cellular/3g/maps/3gmaps/coverage_maps.pdf
http://www.ofcom.org.uk/radiocomms/...and/cellular/3g/maps/3gmaps/coverage_maps.pdf