I've also added a UPS for the internal ONT. I wasn't entirely satisfied with my initial installation, partly because the low-voltage power lead from the UPS looked quite ugly (not helped by being black) and wasn't such a snug fit in the ONT input socket, so I decided to make use of the redundant master socket back box to connect the supplied Openreach lead to the low-voltage output lead from the UPS. The uploaded photos show how the work was progressed.
That’s Top Notch mate.
Thank you. I'm happy with what I have got now; both the package from BT (Full Fibre 500 + Halo 3, for the same price as Full Fibre 100, which I was on for a couple of days initially) and the installation of the hardware. It's taken a few weeks to get there, but I feel it was worth it.