I’ve provided you a helpful technical solution. Other than my joke comment above to candlerb, I was being sincere and not ‘flip’ as you put it.
Now the basic reality of the situation is that there is *no* way any SP is going to provide a standard 12 hour+ backup solution for FTTP - unless they are forced to. I’m not aware that they are.
Indeed Openreach withdrew their BBU solution in 2019, as it happens shortly after I had mine installed. This would be no good to you anyway as it only protects the ONT itself for around an hour. Any other equipment needed for voice over fibre (router and/or ATA or DEct base etc) is not covered.
Other folks including myself put up their own UPS typically to cover outages that last up to an hour or two at most. Beyond that it is taken that the utility will have returned or another source of backup power (generator) is available.
That’s the harsh reality.
The technical solution I described above (a DC UPS) is really nothing more than a slightly more elegant version of a car battery with main charger powering the ONT and router, etc directly at low voltage 5/12 or 20V DC. This is really just an expansion of the BBU idea and is technically feasible and not extraordinarily expensive not difficult. It’s also the only readily available off the shelf way you’re going to approach or exceed 12 hours backup.
What you choose to do with this is up to you. Shoot the messenger. I don’t really give a stuff.