"I have a suspicion that coverage maps are based on old data"
With a good measure of wishful thinking thrown in I think. The coverage map for the area my sisters house in Kent shows unbroken outdoor coverage for my network, with some patchy indoor coverage. The reality is I've never once had a signal within a 3 mile radius.
The networks have long been accused of using out of date maps, which if true would be more of a problem in the case of Tmobile/Orange. Since they merged to form EE, they've been weeding out "duplication" of masts, which according to residents in some of the areas affected, wasn't duplication at all as masts weren't necessarily on top of each other and changes fail to account for terrain blocking signals. If EE maps are still pimping the same coverage without accounting for the lost masts, they're being disingenuous at best.