Worth noting that Hennessy was talking about the time of Harold Macmillan and the V-bombers.

A great book:

http://www.amazon.co.uk/The-Secret-State-Whitehall-Cold/dp/0...

Even in fairly "optimistic" attack scenarios over 50% of the UK population were expected to die immediately in an attack:

e.g. http://en.wikipedia.org/wiki/Square_Leg

For those interested in UK government cold war planning I can strongly recommend Peter Hennessy's book "The Secret State: Preparing for the Worst 1945-2010":

http://www.amazon.co.uk/The-Secret-State-Whitehall-Cold/dp/0...

One particularly chilling part describes how early UK estimates thought that it would only take a small number of H-bombs to destroy the UK as a functioning society, resulting in the following:

"Hennessy tells the story of a jokey 1961 encounter between Khrushchev and the British Ambassador, Frank Roberts. The Soviet leader asked how many H-bombs Roberts thought would be needed to wipe out the UK. ‘Six’, he replied. Khrushchev chided him for his pessimism. He said that ‘optimists’ estimated it would take nine — but reassured him with a twinkle that the Soviet General Staff… had earmarked several scores of bombs for use against the UK so that the Soviet Union had a higher opinion of the UK’s resistance capacity than the UK itself."

For anyone interested in this kind of stuff I can recommend this book:

http://www.amazon.co.uk/Secret-State-Whitehall-Cold-War/dp/0...

The story of how the government planned to communicate with the V bomber fleet is wonderful...