Small cells ? 5G ? Oh please, it is so much simpler than that
Sigh, someone needs to go on a basic physics course.
If you want coverage the answer is (and always will be) a low frequency band + high output power radios. It is that simple and the UK has a perfectly good low frequency band available, Band 20 at 800MHz.
By the way, when people complain that all they can get is 2G or 2G data, then at that spot could easily be covered by 4G/LTE on band 20.
2G/GSM uses band 8 for coverage which is 900 MHz and LTE on Band 20 has the same coverage grid.
So there is no technical nor spectrum shortage for operators to provide LTE coverage on par with GSM, all they have to do is equip their 2G sites with 4G on band 20.
So why don't they ? Economics. It is expensive to build out a site, it is expensive to bring high speed IP links to a site (via fiber or microwave) and often landlords raise the site rental when you add new equipment to the site.
So operators do the absolute minimum to meet regulatory requirements and only add extra sites where it makes economic sense (i.e. where they can cover enough people who are willing to pay extra for a 4G subscription).
If you want coverage, mandate a minimum geographical coverage in % and not a population coverage in %.
Small cells provide capacity, not coverage. Pretty simple physics, a small cell covers a smaller area, a smaller area has fewer people in it and therefore the radio spectrum, which is a shared resource, is shared among fewer people, thus each person gets access to more bandwidth and thus more capacity.
Simple trade off. Large coverage, lower capacity. Small coverage, higher capacity.
However 4G/LTE is much more spectral efficient than 2G for delivering data, so even a "low capacity" but large coverage LTE cell on band 20 can easily deliver 20 to 50 Mbps of data connectivity Vs 0.2 Mbps (200 kbps) on 2G.
And 5G ? 5G will be in high frequency bands and is all about capacity. The lowest 5G frequency band will be 3.5Ghz (3500 MHz) and the other bands will be up around 15, 28 or even 60 GHz.
High frequency bands are fantastic for capacity, but hopeless for coverage. So about the only thing 5G won't be solving, is coverage.