Comment by arethuza

One thing to note is that the UK had a huge number (over 1500) small underground observation posts operated by the Royal Observer Corps - in the case of war these were to monitor nuclear explosions and fall-out:

https://en.wikipedia.org/wiki/Royal_Observer_Corps_Monitorin...

This short film from 1971 that includes scenes filmed in observation posts and a central bunker:

https://www.youtube.com/watch?v=o3y0WmONP4E