not to mention any original iphone 5 won't work on O2 4g.
Fixed to clarify. Later iPhone 5 models, such as 5S and 5C should work fine.
As for coverage, yes its currently poor on all networks. However, part of the licence agreement is they will provide 98% population coverage with 4G by 2017 (I think that's the right date), following the utter balls-up OFCOM made of the 3G rollout requirements.
Somehow, I get a nagging feeling that O2 would rather pay any penalties than increase coverage outside of Londonium and other large conurbations, based on their lack of willingness to increase their 3G coverage over the past few years (and it's believed that O2 mostly used base units that just needed a firmware update/enablement to provide EDGE and 3G, plus the associated increase in backhaul capacity).
For us poor souls that need near universal coverage for calls, there is no other viable option around these parts