[Yorksman]

They have a facilities at Millbank and elsewhere as shown on their website:


I doubt they'd struggle unless someone dropped a nuclear bomb on London, and then only briefly, as I'm sure they could connect from a Bureau somewhere in the world
(assuming there was anyone left elsewhere in the UK to receive it).   

There was an incident back in 2010 where GIR lost power and Julie Etchingham did a truncated News at Ten from a broom cupboard in Millbank.

I remember that and on that night ITV were showing an FA Cup match between Liverpool and Reading, and Reading scored in 2nd half injury time, so the game went to extra time.

I think behind the scenes they were glad it went to extra time as it would have been chaos if News At Ten had been on at 10pm.

Edit: Here's the thread from that night, from the old blue place:
tvforum.uk