While shooting the movie, the BBC got in trouble with local police when they detonated a large smoke bomb in order to simulate a nuclear explosion. Members of the public, who were not aware that a movie was being shot, panicked and thought that a real explosion had occurred.
In the aftermath of the nuclear blast, footage of a cat supposedly "suffocating" outside in the extreme heat is shown. This is actually footage of a cat enjoying a hefty dose of catnip, then they just reversed the film to give the impression of the cat suffocating (the way the cat is rolling on the ground is the giveaway).
Findings from the 1980 British Government exercise "Square Leg" were used as the basis for projecting the level of destruction and number of casualties in the movie. "Square Leg" was a government project that estimated what would happen in Britain in the event of an actual attack. It projected the mortality rate at 29 million, serious injuries at 7 million and short term survivors at 19 million.