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.