Expat in NorthEast Lancanshire. FreedomPop has excellent prices and different plans. O2 and Vodaphone are a bunch of thieves, the first thing they will do is to sell your emails addresses to spammers. Also their plans are rubbish.
Network is quite good everywhere for all UK telcos (except maybe in remote areas or little islands). On the other hand customer service (O2, vodafone) is really bad and unprofessional.