This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / When the Wind Blows
The animation used to depict the bombing- showing people being vaporised by the heat and shockwave of the blast, sheep tossed across the countryside like balls of cotton wool, a train blown off the tracks, crashing over the side of a bridge and into the river it's crossing and houses blown apart- all to an utterly chilling and violent musical score underlying it.
When the Bloggses think they can smell roast pork, it's heavily implied to actually be the seared flesh of dead people, cooked by the heat of the blast.
Once the bombs have dropped, the Bloggses' naivete creates an incredible sense of horror simply by being juxtaposed against the post-nuclear wasteland.
The fact that theeventsdepicted in this film are still a very real possibility, even to this day.
The end of the credits where it pans over the dedication to the children born to the crew during the development of the film (and their futures), there is a type of Morse Code being played. This code stands for M.A.D.- or, Mutually Assured Destruction.