This is a really tough one I reckon…..
At school in the late 80s, we were shown a video in class about the after effects of a nuclear attack.
All I can remember is that at the start there was a man who was naked on the moors, he was running and I think he fell into a stream, where there was a dead sheep.
Told you it was a tough one 🙂
“The War Game” (1965)?
https://www.imdb.com/title/tt0059894/?ref_=kw_li_tt
Just a guess.
B&W or color?
American, English, or other?
Threads
“Threads” was my guess if a color film; “The War Game is in black&white.
Threads doesn’t open with a scene like that, although it may have one near the end.