He is supposed to determine whether Britain is still functioning on the basis of whether or not the Today programme is still broadcasting. If it isn’t he has to retrieve from his safe the PM’s letter instructing him to: 1. Put himself under the command of the USA, if it is still there. 2. Make his way to Austrailia, if it is still there. 3. Get on with it and take out Moscow. 4. Use his own judgement.
Source: p. 17 of LRB 2002-08-08 in a review by R.W.Johnson of Percy Cradock’s Know Your Enemy: How the Joint Intelligence Committee Saw the World and Peter Hennessy’s The Secret State: Whitehall and the Cold War.