Chris, at Channel One, NZBC, in the 60s and 70s, videotape was expensive (2inch wide) and at 15ips, a half hour programme on a metal spool was heavy and took up lots of space.
So unless a producer said "Keep that one" (in which case someone had to find storage space) after a certain period of time the videotape room was instructed to ERASE the tape and put it back in the shelves to use again. Some of the news or sports stuff that they wanted for the archives, was fed through the Telerecorder and a black and white film was made - usually very grainy and of lousy quality.
The methods of recording used today were only a dream then.
I'm sure BBC had the same problems. If a programme was done on film, there was a solid copy, but if it was a studio programme on VT, the tapes were usually wiped. (Except for Wn3366 which I kept hidden behind #1)
Robyn ex WNTV1 VT and Telecine operator, 1968 - 1973