Might this be an air raid shelter in Wolverhampton ?

The Old main post Office in Wolverhampton City Centre is one of those buildings in which a basement is separated from the pavement by a gap ,on either side of the promontory shown in the first picture part of the gap has been covered by a thick concrete roof the second picture is the left hand side ,and the third the right ,the construction reminds me of the above ground air raid shelters i have seen and i suspect that this unelegant addition was an air raid shelter .

That is one of the many purpose-built Post Offices and probably pre-dates even the First World War. The design would have incorporated substantial basement rooms so, perhaps, as part of the Civil Defence preparation for World Wat Two access to the basements was provided by external modification to add two entrances with a reinforced concrete roof slab above.

I recall that these structures were steel framed with masonry infill so could be considered safe havens in a bombing raid.

Next time I go into Oxford I will check that original GPO.

I reguarly walk past the old Post Office in Preston and had wondered why it had a basement, I think I know why now.

The Preston Post Office was one of the many built to a similar design and opening in the early 1900s. Basements were planned and built for post office functions such as sorting and not as refuges.
The Air Raid Precautions Act of January 1938 resulted in local authorities being placed under a legal obligation to prepare air raid precaution schemes.Although this resulted in the construction of purpose-built shelters, existing buildings were considered and I believe these solidly built POs with existing basements were utilised in some cases.