They're on another planet
If you haven't been out of London for a while, it would be easy to believe coverage is good; in my experience its rare to find a spot indoors or out where the signal is poor or non-existent. Take a trip outside the M25 and its a different kettle of fish altogether. Even largish towns which I would have thought would have been easy wins are peppered with holes, and heading into the countryside - even the well populated south east - and a decent signal seems the exception rather than the rule. A look at most coverage maps based on actual use shows good coverage near roads and thats about it, but that might well be down to testing method.
Ofcom is pathetically fond of asking questions that appear guaranteed to provide answers that show it doing a good job, at which point it pats itself on the back. The only thing that seems to change that is someone else independently producing facts and sharing them loudly and publicly, which occasionally shames Ofcom into into a bit of Doing The Right Thing, albeit the 'lite' version.
Real world data coverage, particularly 3G/4G, makes voice coverage look positively stellar, so it'll be interesting to see if the forthcoming report is as much self-serving whitewash as this one.