This attitude is part of 'better to be dead than a slave'. I think many people saw what the Nazis did in the occupied territories and their thinking was influenced by that (extending it to, what would happen if the Soviets invaded our lands).
By the 70s and 80s, people knew (or had the option to know - many chose not to know) what the Soviets did to people at home, so it wasn't totally out of whack to assume what would happen if they invaded.
People often forget how awful the west treated the bottom of its society at the time. For a black in the South, the soviet union (or more likely Cuba/communist latin country) probably looked pretty good.
That's not to excuse or downplay these country's mistakes. Let's just not forget our own.
Well, had it been my choice between a nuclear war where 90%-95% of the population of the UK would have died in optimistic scenarios and surrendering to the Soviets (and yes, I am fully aware of how badly they treated people in their own country and the countries they controlled) I know which one I would have chosen.
It was between maintaining the capability of such a nuclear war to deter aggression, and not maintaining it and surely having been invaded by the Soviets.
For the madness that MAD was, for all intents and purposes, it actually worked.
That's the point of deterrence. It didn't start. The moral thing to do if it started would have been to not fire the missiles. But any leak or hint of such an intent would have had disastrous effect on the deterrent, and is thus an extremely dangerous idea for any leader to actually entertain. One can hope that leaders would have privately decided against and that the letters of last resort were sealed and destroyed for that reason.
Utter craziness - all of it.