alt Hacker NewsThere'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".
I used to be a first responder with a Firstnet setup (not just the plan discount, but the actual black SIM card) that could roam AT&T to Verizon to TMO as needed, so was as close to universal connectivity as feasible. Though (probably relatedly) it was always 1-2 generations behind (many areas were still ATT LTE, maybe 5GE, when they were rolling out 5G).
And the clusterfuck when I tried to transition my account back to normal, where an $8 balance that wasn't reconciled triggered the suspension of my AT&T whole family account, but when I tried to pay, no-one in FirstNet support or AT&T could tell me how much to pay or where or my account number (and this is in the store), until a poor store and a poor phone CSR spent THREE HOURS getting it resolved. "I am literally trying to give you the money to take care of this." "We don't know where to have you pay that money to fix this."
I was an early adopter, but FML.