Last night I watched "No Blade of Grass". a 1970 British film produced and directed by Cornell Wilde.
The theme was environmental destruction (fueled by pollution, etc.) which resulted in some kind of plague which was poisoning the worlds' food supplies (grain, etc.).
It involves an architect and his family who flee a collapsing London and head north towards his brother's farm, and the troubles they encounter on the trip thereto.
If you can get past the "man is totally polluting the world" part of it, it was actually quite good and pulled no punches.
I don't know how successfully this can be found through normal channels.
But it CAN be found here:
tpb22.ukpass.co/description.php?id=21171456
(you will need to add "https://" to the beginning of the link above)