A quick look at 'who owns O2?' will show you a corporate merrygoround where financial engineering is more important than improving the network on the ground ie. very serious infrastructure investment.
Telefonica prefers to take money out than invest the BILLIONS they don't have. And probably cant raise (at reasonable terms) even if they wanted to.
THEN we add the UK governments obscene TAX on the nation to acquire a network licence. Those BILLIONS companies willingly paid for a licence to rip consumers off are being paid for by GUESS WHO??
Chinese network gear is 'verboten' so unless Erikson or some other European suppliers step up to the plate with deals UK networks can afford (after bleeding their cashflow dry) better service and coverage is a pipedream.