The UK 3G coverage is pretty good compared to other nations, it's not perfect though. A lot of expansion is halted due to locals getting masts refused/resited rather than lack of investiment. 3/T-Mobile seem from my experience across the Scotland & England. Orange and Voda, then O2.
(I travel lots have an Orange iPhone, 3 Dongle & Voda iPad and until recently had a O2 works phone.
Orange & Voda are pretty hard to beat with '2G'
Not long until 3/T-Mobile/Orange 3G Super roaming Network is activated... Maybe 28th of April as it will be Orange's 17 year anniversary!