Re: What about UK ?
> By its very nature and frequency range, 5G needs more masts (many thousands) and Micro cells to be a meaningful deployment
You are ill-informed. For sure in the high frequencies (e.g. >6GHz) this is the case, but 5G is currently being deployed at much lower frequencies so that coverage matches the underlying 4G network (and doesn't give the highest data rate or lowest latency which is possible at the high frequencies). Even with a bit of disparity, though, the addition of beam forming on the 5G layer gives more range gain to make it a match in range with 4G. Lastly, you will not only be on 5G, but also on 4G at the same time.
Caveat - your "meaningful deployment" may not be what I'd call a meaningful deployment. You might be demanding ubiquitous gbps coverage, which is unreasonable, imho.