In Essex, there is a series of brown road signs labelled 'Secret Nuclear Bunker', directing you to a Cold War bunker in Kelvedon Hatch that has been turned into a visitors centre complete with mannequins and spooky radio and siren recordings to set the mood.
Could it be used if a Nuclear threat was imminent (aside: I hate that this is a more genuine concern than it has been for more than 30 years)? It's probably more likely to be stormed by regular people with garden tools for protection.
The amount of secret infrastructure there must be to maintain military and governmental concerns during war is vast. I wouldn't be shocked to learn there are vessels ready to go to space, packed with Tories. Ironically they make it to some distant habitable planet only to be turned back at the border by a bunch of xenophobic ETs.