Came across this the other day on our official walk exercise for the day. Looks to me like an Anderson Shelter, these were made by the Government and were intended to protect the population of London from the Nazi Bombs that rained down during the Blitz. They were not altogether that effective, were easily flooded, damp and very uncomfortable I have read recently. They did help lift morale as they gave the impression that the Government was doing useful things to help its people. Where have I heard that recently.
This model now sits in a field not far away from where we live, so it has at least lasted. Then again I could be wrong, it might just be a pig pen.