In reply to imkevinmc:
However, mobile phone companies say this would remove their incentive to build more masts to improve coverage.
Yeah, like that's a priority for them just now, rather than 4G rollout (which anecdotally is also leading to reduced 3G coverage).
One theory is that, since 4G has better penetration inside buildings, the networks are taking the opportunity to cut down on the number of masts they have to maintain. That could mean that coverage for 3G users is going to get worse.
http://www.theguardian.com/money/2013/dec/17/mobile-phone-coverage-4g-3g-si...
The above linked article also states that Vodafone and O2 have a mast-sharing arrangement. It's not clear whether that's just a real estate arrangement (ie each can put an antenna on the other's masts), or they actually allow traffic from the other network through their anntennae. This article from The Register seems to suggest it's the latter:
http://www.theregister.co.uk/2012/06/07/vodafonica/
So in practical terms, there
should be little difference in coverage between Vodafone and O2.
To the OP: if you're happy with the performance of the Vodafone network but don't like their prices or customer service then you could consider changing to one of the virtual providers that uses the Vodafone network. I've just done this, moving to Sainsburys and porting my number with no hassle at all. (I didn't even get referred to the retentions department when I called Vodafone to get my PAC - perhaps they were glad to see the back of me?!)