It was a sort of spin-off from an episode of the BBC pop-sci documentary strand QED. A few years before Threads they did an edition called "A Guide to Armageddon" in which they descibed in detail what would happen to London if a bomb was dropped over St Paul Cathederal. They also used make-up to show the sorts of injuries people would receive. A lot of the people involved in the documentary went on to work on Threads.
It's on YouTube if you want to check it out.