Just coming back to this, so i've decided to do some testing and despite me saying I wouldn't touch RWG .. in the end, I did

I decided as long as I don't port any numbers to it I'd take the risk and give it a try. but I have to say the reviews about their support are true. Zero responses from them. At all. Thankfully despite them saying you have to email to activate it, it worked immediately as soon as I put it in the phone.
Anyway, I also decided to go for the anywhereesim as well. These are the only two that I could find at the time that seemed to be UK companies and also covered all 4 networks.
So here's what i've found and a small comparison
Both SIMs will join a network, however the RWG SIM always wants to join EE. It doesn't matter if there is a stronger signal with another network, if there's an EE signal, it will join it. Trying to force it on to another network fails each time. I can't explain this, I know there is such a thing as it being "steered" but I would have thought I could manually select a network but not from what i've tried. EE if it's available. Otherwise it will join another network if EE is not around. I've not been able to test out Vodafone because the places i've taken it caused it to join either EE or O2.
The network appears as EE Tele2 IoT
Data is slow as ... very very slow. A check on whatismyip etc comes up as some 3rd party hosting company (forgot which, will check and update later). But it does eventually work.
I paid £30 for 12 months. 1GB each month. No 5G.
Now on to the real winner I think: anywhereesim.
Able to swap it to any network I want. No problem.
Added ability to make calls (RWG is data only)
More expensive at £50 for 12 months.
Able to select any network.
Data speed doesn't appear to be capped, but im not going to test this extensively given I have limited data. But much faster than the RWG SIM. Even on the same net
Downside: they've also not responded to my pre-sales questions. Not support, but pre-sales. not a good look.
But at least my wife and I now have a SIM that should give us emergency data/calls via VoIP if there's a working network of any kind nearby.