"[...]
a dramatic account of nuclear war and its effects in Britain, specifically on the city of Sheffield in Northern England.
[...]
A third and final attack targets primary economic targets such as the Tinsley Viaduct. This third attack causes massive structural damage to Sheffield; the blast and heat kill an estimated 12 to 30 million people in the U.K. in the wider exchange. [...]"
Had to look it up, seems the movie was seen as optimistic ...
https://en.wikipedia.org/wiki/Threads_(1984_film):
"[...] a dramatic account of nuclear war and its effects in Britain, specifically on the city of Sheffield in Northern England. [...] A third and final attack targets primary economic targets such as the Tinsley Viaduct. This third attack causes massive structural damage to Sheffield; the blast and heat kill an estimated 12 to 30 million people in the U.K. in the wider exchange. [...]"