I've heard from friends on the inside that O2 and Vodafone were about to rebuild their networks and share the UK as one physical network (I don't know if this is still true). If so, there could be positives and negatives to both O2 and Vodafone in the near future. A new physical access network will have teething troubles. If they alter their existing 2G networks then even voice calls could get tricky to make in congested areas if they've decided to "fully realise the synergies of cost savings" possible by amalgamating their networks together. I gather Vodafone will manage one side of the UK and O2 would manage the other.
Could be worth a "wait and see" on these two for now.
edit: sounds like they're actually extending their networks in a joint fashion to help coverage - 17,500 extra Base stations.
http://www.vodafone.co.uk/our-responsibilities/our-network/
vodafone state it doesn't alter their existing spectrum, etc so I guess this would be a 3G / 4G coverage exercise. Could be that Vodafone or O2 are a good bet!