TIL that the BBC had no archival policy until 1978, and many of its old broadcasts were only aired live. A number of BBC broadcasts are lost, including a show featuring a then-unknown Bob Dylan; 97...
They are still finding new ones though! We may never see all of them, unfortunately, but we can see a large chunk of them. That's something to be grateful for considering the circumstances.
A lot of the 2nd Doctor is missing, some of the 1st. Some episodes have been animated, but some are only audio and still frames. It's a bit rough to get through.
I was watching the Monty Python documentary and they mentioned this. I think one of the Terry's bought the episodes, which were scheduled to be erased. Otherwise, no Monty Python for America. What was remarkable was many shows that the Pythons had written for, and the ones that had influenced them, At Last the 1948 Show or the Goon Show were gone. So in an alternate timeline, we might be huge Spike Milligan fans.
In the lost media community there is a distinction between things that are known to be lost forever (baring some miracle) and things that may still exist but either no known copies have been found or the owner refuses to release a digital copy (usually due to copyright reasons). For example most of the early seasons of Doctor Who were lost due to the BBCs policy but over the years copies of the film and even some rudimentary at home recordings have been found, allowing for them to be archived and preserved
There are similar examples from other countries as well. TV was seen as a means for making live broadcasts, so back-ups weren't made and tapes got recorded over. Source: a course on TV history I took.