Absolutely no reason why they should have failed, I'm staggered to discover as a result of this incident that they don't use Buffer sets and UPS systems to hold at least all the IT kit up during a power failure. Ensuring the whole building shouldn't take much UPS gear too. If they are saying that the standby power plant did in fact start up, synchronising that with the UPS buffer supply should have been a piece of cake too.
Do we know that their IT infrastructure WASN'T running on UPS(s)?
It wouldn't surprise me if the core back-end servers for ENPS
and their Avid editing system (which is accessible by ITV News operations around the UK) were still up and running on UPSs, but that there was no power for the newsroom PCs that the journalists actually use, possibly the Avids that were doing the edits, and that the gallery stuff (vision mixers, lighting desks, sound desks, lighting, virtual set etc.) wasn't powered (and then rebooted when it was)
Alternatively, ENPS has "buddy server" functionality, which many operations use to background synchronise main servers with off-site backups, so that routine maintenance can be done on servers (by switching to the buddy) and that you are covered if the main server falls over unexpectedly.
As such this would mean that Millbank may have been able to access some running order/script content, and if the Avid distributed editing system servers were still up at Grays Inn Road, they may also have been able to start exporting VT
content (possibly not finalised?) to Millbank's local Avids for play-in?
As for routing the Millbank studios to Chiswick/Leeds (and Glasgow / Belfast) then that is just basic lines bookings (probably via BT Tower as an option, as evacuation centres are normally also set-up to be capable of being independent of internal lines infrastructure - in case that goes down)
When the BBC lost power at TVC, the newsroom was running on UPS, and the first thing that happened is that every non-essential TV, and non-essential PC, was switched off (including monitors in the gallery that weren't needed), as was the studio lighting.
Back-up power is a tricky thing to engineer without a UPS in the way, you inevitably get an interruption.
In the days of analogue gear (and early 80s simple computer stuff with ROM-based OSs that booted fully in seconds), that could mean just a short drop-out of the programme (less than 30 seconds?), but now in the days of computer based stuff, this can mean 5 minutes or so before everything is back up and running.