The shocking thing about 'Protect and Survive' is that whilst we poor mugs sat under our lean-to shelters trying to stay out of the fallout, our esteemed bureaucrats would be cowering in their underground citadels with food and air con.
If a nuclear exchange took place in 1980 between NATO and Warsaw Pact, it would have been, by all accounts, devastating. 'Protect and Survive' was meant to try and reassure people that they could live through an all out nuclear war.
People weren't that stupid, and knew that a nuclear war would mean the end of their world, even though the planet would still be intact. The end of their world means millions dead within 12 months. No more foreign holidays, no more fresh food for a while. Ecological catastrophe, disease, hunger, deprivation, probably martial law and a number of other less pleasant burdens.
Parallels with Hiroshima & Nagasaki are not valid comparisons with the estimated scale of a nuclear war in the 80s. Those were 2 small bombs, Kiloton warheads. During the 80s, there were hundreds, if not thousands of warheads, each an order of magnitude more powerful than Little Boy or Fat Man, pointed at targets accross Europe and Asia.
'Protect and Survive' outlined a futile plan as a palliative measure for a populace that was largely doomed to die in the event of a nuclear confrontation. Interestingly it was never issued, and remained for sale only.
If they had popped it through every letter box in the country, mass panic would have resulted, so they were avoiding doing so until they absolutely had to.