Here’s a hack, get yourself a cheap eSIM data only plan from an alternative UK network (VOXI, Talkmobile etc) if you main network doesn’t have connectivity; they will!
There's even eSIMs specifically marketed as being a "backup" esim, with coverage on _all_ UK networks.
At least on my android, you could set the second esim as a "backup" that it would switch to for data if the main one lost connection (it took a few seconds, so it wasn't an "always connected" experience, probably because the phone wants to save power)
Lots of options if you search for "esim UK all networks".
Or if your main sim is an eSIM, there seem to be 500GB/month business sims for effectively £2 a month online. You can't port your existing mobile phone number over, but it's fine for data.
I was introduced to it by another HN poster: https://www.hotukdeals.com/deals/three-500gb-preloaded-5g-da...