That's my recollection anyways. I read it in 6th grade, so it's been well over a decade since I read it. I really ought to re-read it.
So, I'm sure that despite the effort the UK Government was well aware of its futility.
http://www.independent.co.uk/news/uk/this-britain/in-the-eve...
Yes - that AA - the Automobile Association....
http://www.huffingtonpost.com/2013/12/05/nuclear-missile-cod...
There are maps from the early 1950s - pre H-Bomb - which show large areas of the UK reduced to radioactive desert after a relatively small attack.
Unloading tens or hundreds of larger weapons on the UK would do very little to increase its survival prospects.
From that POV, Threads is terrifying but optimistic. It's not clear anyone at all would survive a full exchange. Spending a few years in a bunker would simply delay the inevitable.
All of life could be said to be merely delaying the inevitable.
I am reminded of the story of a man running away from a tiger when he reaches the edge of a cliff and falls over, managing to grab hold of the edge and hanging by one hand. Below him is certain death from the fall, above him is death by the tiger. The man notices some sweet dew dripping from a blade of grass by his face. With moments left to live, he licks the dew.