In reply to a post by jpm:

I'd like to see real-world photos of this
https://www.businessdirect.bt.com/products/cyberpowe...
BT seem to be claiming on that page that it powers the ONT and a Smart Hub 2 and they have a non-FTTP version of the UPS so I assume there's a difference.

Mmm the guide linked has illustrations so you can see it's DC output on the back:
https://www.businessdirect.bt.com/content/uni2/docum...

It looks like this:
https://www.cyberpower.com/uk/en/product/series/indo...

Specs say it's 36W and 1.5 hours so I presume that's 54Wh which isn't the worst but neither is it the best.
They say it takes 20 hours to recharge from 0% to 90% on promo with spec sheet saying typical time is 14 hours so I'm not quite sure which to believe.. If it can deliver 1.5 hours at 36W but actual usage is probably much less, it might actually deliver over 3 hours if it's well designed. Obviously would need to test to know.

The one thing to note is this has one output.. unless you have a splitter or there's some pass through (don't think may ONT has one) then I'm not sure how it will power the ONT and modem?