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 death metal score underlying it.
When the Bloggses think they can smell roast pork, it's heavily implied to actually be the seared flesh of the dead, 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 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.
While the movie does depict the effects of radiation poisoning quite well (showing that the Blogsses become very thin, develop sores on their body and Hilda losing her hair), the comic is far more chilling. The two of them basically lose all the colour in their skin, become visibly ill almost right from the start and Jim even starts bleeding from his mouth while singing a song trying to cheer Hilda up.
There's something disturbing about the scene in which Jim finally shows that he is afraid for his and Hilda's survival when he realises that they have no water left.
Jim:[weakly] Put the kettle on, will you, ducks? Hilda: There's no water left, dear... Jim: Oh yes, of course. Well...we'll just have to have milk then. Hilda: That pint's gone bad, dear. The fridge has been off. Jim: Oh...heck. B-black coffee, then... Hilda: There's still no water, James. Jim stands up and stumbles over to Hilda Jim: Well...what are we going to drink?! Eh?! EH?!(grabs and shakes Hilda) What ARE WE GOING TO DRINK, FOR GOD'S SAKE!?