Living in the Chilterns
It always surprises me how bad reception is this close to London, and with so many population centres. When you do get a signal it's usually 2G anyway, Edge if you are lucky. I live about 1/2 mile from my O2 transmitter, and get a very poor signal. 5G won't make any difference, I can't see any network bothering with it outside urban areas, at a guess signal attenuation will be even worse, and reduce effective cell sizes even more.
Maybe the ministry of fun could look into the provision of community masts, run by say civil parish councils, and available to all network providers. For parochial parish councils, you can get some very discreet masts for your church towers to help with the steeple fund.
I suspect the hard to get to places will always have poor signals without public funds, but why is the signal round the M25 still so bad.
PS - Geography is one thing, but there are such things as Micro and Pico cells for blind spots, and motorways and railways tend have network access along their length to the emergency phones if nothing else, so cable runs shouldn't be a problem, although the latter might have a problem with safety cases.