It's a stupid idea
Coverage just isn't up to it anywhere vaguely rural. Even in the relative flatlands of East Anglia, you'll find loads of areas of marginal coverage. Even "villages" of 3000+ people where indoor/groundfloor coverage isn't sufficient to make/not-drop a call. Yes, areas without even reliable 2G voice/SMS coverage on EE.
It's mostly not a case of adding a few more masts, more that there's all sorts of random deadspots because the coverage of one mast doesn't reliably overlap with the next.
And when a cluster of masts in an areas stop working for a week or two at a time (happens every couple of years in my area with EE) then various spots have no outdoor coverage either.
The phone system is incredibly complex with many interconnected systems, and we know full well that large numbers of people across huge geographic areas lose connection for 12 hours or so several times a year, because some server failed.
The basic requirements are fundamentally different.
I think the case made by "x 7" about resilience in the face of longer-term power loss clinches it though. The power needs to keep a dedicated emergency-services mast going, which only operates a few channels intermittently is VASTLY less than a commercial phone-mast (and probably is plausible with a modest battery). No amount of "high priority SIM" helps if the cell-tower batteries have already been run down by Joe Public Facebook 'live' -streaming videos of the flooding or whatever.
For the core, super-resilient service, you want a dedicated system, probably at VHF, to get comprehensive coverage, including indoors, from a modest number of towers. The core system MUST be a 'professional' system and stand apart from "consumer crap".
By all means use commerical networks for non-essential high-bandwidth nice-to-haves (photo/video uploads, face-recognition, whatever) accessories, maybe with custom apps on regular phones.