This title was wiped as a cost-saving measure
Firstly, tapes were much more expensive in those days - hundreds of pounds for a single tape, not twenty quid. When my dad was at Grampian in the early seventies, the station's *entire tape library* was about two dozen one-hour tapes.
The second reason was that Equity was against repeats - unlike a stage show, a TV show could be repeated without involving any actors, depriving them of their livelihoods. They thus had high fees negotiated for repeats beyond an initial few showings, which means that your comment about reshooting for £50K is closer to the truth than you may realise - with the amount a repeat would have cost, you might as well shoot something new.
In the BBC's case this was exacerbated by the fact that there was a conflict between two departments - the Engineering Division, who recorded the program on videotape and stored it until broadcast, and BBC Enterprises, who transferred programs to film (because it was cheaper and universally-used) to flog to other countries. Both felt that long term archival was the other's job, and thus neither invested in the infrastructure to store programs in any format. As a result, stuff just got wiped or thrown out - by Engineering because they wanted to re-use the tape; by Enterprises because they'd sold it to everyone they could, and didn't want cans of film cluttering up the place.