Where have all these men gone?

UK male labour force participation is perplexing.

Back in the early 1970s, virtually all UK males aged between 16 and 59 were either working or registered as unemployed. Only 800,000 were economically inactive. Today, that number of inactive males is around 3.5 million.

What are these men doing? Are they all students? Surely not. They are not not registered unemployed, because if they were, they would be regarded as being economically active.

If anyone knows where these men have gone, then please let me know.