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Nightmare Fuel: When the Wind Blows
The animation used to depict the bombing, showing people blown apart, sheep tossed across the countryside like balls of cotton wool, a train blown off the tracks and into the river it's crossing, houses blown over, 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 this film is a very real possibility.