Your comment on “BBC using two kinds of film” brought a chuckle. Interior shots of that era were shot on 50 fps video tape, making a nice crisp view when transferred to 50 fps video for broadcast. Exterior shots were shot on traditional 24 fps film, making for a deeper contrast, but a bit of a jitter when transferred.
There’s even a skit of Monty Python that makes fun of this. Google for “Monty python” “surrounded by film” for links.