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".