When younger I worked for British Gas. Many basements had square corrugated steel cladding on the ceilings propped up by what looked by Acrow jacks. The idea was to support the collapsed building above. A direct hit would smash most air raid shelters.

The shelters were around until the 1960s. They blocked them up. They stayed as they were too expensive to demolish - or expecting another world war. "Large" brick and concrete structures behind tenements and blocks of flats. As a kid we would run across the flat roofs and jump from one to the other.