Well, count yourself lucky to get a signal *ANYWHERE* with Three. They have pretty poor coverage. The real networks - Voda, O2, EE - do have pretty reasonable outdoor coverage.
The UK regulator, OFCOM, are totally inept in every way. Cynics might actually go as far as saying corrupt. I'm kinder, and think its just they are incompetent.
For the licence auctions for both 3G and 4G, there was a demand as part of that that they had to provide 95% (from memory, might be slightly different) coverage. Being inept/corrupt/incompetent, they never specified what that meant, so all mobile networks took the easier option of 95% population coverage, not 95% geographical coverage. And OFCOM never even bothered to enforce that, too busy meddling elsewhere due to being inept/corrupt/incompetent. EE is, for other statuary reasons, aiming for 100% outdoor geographical coverage on 4G (not, 4G cannot do native voice calls, you handset will drop back to 3G or 2G, or attempt VoIP instead if your handset and provider support it).
It is catastrophically expensive to build a mobile network, and catastrophically expensive to run it, and UK consumers don't want to pay anything for the network element (but are happy to pay a grand for a shiny handset - go figure). Proper, large 5G masts are going to need at least a 10Gbps backhaul per site, which is catastrophically expensive. Its proper big money.
So its only natural, like 4G, 3G, GSM or even TACS before it, its initially deployed where there will be lots of early adopters. Cities. No point putting the first ones in a remote valley where you may at best get 1 or 2 users, as putting up an expensive mast, then paying a horrifically high rental on your fast backhaul. Clearly that's financial suicide. You need to get it to the masses first.
As to 5G, I wont be an early adopter. Its unlikely to come to Brackley in the next couple of years (although cell tower software updates could potentially be applied, that doesn't solve the backhaul issue - one of the reasons O2 were so slow to roll out 3G), and I have no plans to update my ageing handset for a couple of years, due to cost. The combination of my handset, an iPhone 6S, and my provider EE is good enough for me. Its incredibly rare I can't get a good 4G outdoor signal. And 4G is plenty good enough for me - Christ, I've had an Ookla speedtest show it over 140MBps before (granted, at Wembley Stadium, obviously one of EE's showcase sites). However, I think the technology available added to the desire for more ISPs to tie up with mobile operators and build 5G femtocells into consumer routers and use the fixed broadband lines as the backhauls, could allow for better mobile coverage in rural villages. But obviously the hype is all about its speed speed speed… ...and I think that mises the point.