I am thinking of getting an old fashioned non smart phone and using a SIM card which can access all mobile networks for hill emergency use.
I will rarely make calls so monthly cost is more important than cost per minute.
Onesimallnetworks.co.uk looks good at £1 per month.

Does anyone have any knowledge or suggestions for this?

Many thanks,
Dave

All phones can access all networks for 999 calls. Even without a sim.

I used to use 3 pay as you go and just stick £10 on at a time. It doesn’t expire and the calls are only 1p / minute or something. Like jam bo says they all access 999, bit it’s handy to have some reserve credit for non 999 emergencies.

SIM card which can access all mobile networks for hill emergency use.

If this is all you want it for you’d be better of with a personal locator beacon (PLB) as you may have issues with phone coverage in the hills.