PN - do I understand your comment r/e streams of missiles or a detonation as meaning dispersal might only start if missiles were observed in flight incoming, or a warhead landed in the UK?
No. I was suggesting, slightly tongue in cheek, were just two of many warnings and indicators. Recalling one's ambassadors is a potential early one although they would more likely to have been sacrificed.
The two examples I cited were at the extreme end on the spectrum but were in no way absolute indicators of the outbreak of hostilities. A nuclear detonation may well have resulted from an accident and as for the stream of missiles, I know that has happened at least once.