There are a lot of known lost episodes out there that collectors saved from thrash. The BBC knows it, everyone knows it, but the collectors won't come forward because they are afraid they are going to be prosecuted. They basically stole property which was meant to be destroyed.

https://www.theguardian.com/tv-and-radio/2023/nov/11/lost-do...

The cost to make a digital copy from film stock has gone way down, to the point that fan groups [1][2] frequently encode and clean up old copies of film:

[1]: https://www.thestarwarstrilogy.com/project-4k77/ [2]: https://www.youtube.com/c/kinekovideo

This of course has various IP implications...