It is tricky
I used to work in the industry and most operators have extremely hi res "predictions" of coverage based on things like land usage, terrain or building height and maybe ray tracing.
Where all that falls down is real life.
So, operators also do "drive tests" where they drive/walk around and measure signal at each location.
A suggestion above was to read signal and location on mobile devices and publish it back somewhere. The problem with that idea is that generally even GPS accuracy is not good enough to distinguish between being inside a metal building with zilch coverage and stood outside with lovely coverage. Or someone holding their iPhone in the wrong hand and someone with a different phone with external antenna.
Even getting the google maps car to measure the signals isn't going to be a good guarantee, except for people that stand in the middle of the road to use their mobile.
As soon as anyone publishes a map with any accuracy, someone will come back and point out the inaccuracies because their phone doesn't work in that location at that instant. Then you're in the game of managing customers, which no doubt OFCOM aren't interested in.
("Your map says I should get signal at my house and I don't" "Really madam, do you live in the cellar?" "Yes, I only bought it because your map says I'll get coverage".... that's just not OFCOM's game)
Ultimately, the vague maps are more useful because they make you realise how vague the promises really are. You think "probably I'll get coverage" or "probably not, lets hope" rather than "If I were to step 3 feet to the left I damn well will get coverage or someone will feel my wrath" which just isn't true, no matter how much measurement/prediction is done.
All that is compounded by things like "cell breathing" where cells with high load can contract to hand off traffic to less loaded cells. So the boundaries change depending on how many users there are. A measurement at 3PM outside a football stadium during a home game will show completely different results to the same done when the team are playing away. That sort of thing can have effects for quite a large radius.
So the answer is : there is no answer.