most seem to be game shows, and such. but i had no idea that bbc and other networks would be so dumb as to deliberately destroy the stuff.
i thought we were losing stuff to disintegration, and at least the one fire.
not to stupidity.
well, we can certainly say that now, with the benefit of hindsight, but we must remember that at that time (the late 1960s), things were quite different: for a start, home video was still far from being a reality, so there was no "market" for old TV outside of reruns.
Also, in the UK, the actors union Equity had fought with the TV companies over recording (and repeating) televised productions. Although Equity could not prevent recording altogether, it was able to add standard clauses to its members' contracts that stipulated that recordings could only be repeated a set number of times within a specific timeframe (usually just a couple years or so), and the fees payable for further use beyond that were deliberately so high that broadcasters would consider it unjustifiable to spend so much money repeating an old programme rather than making a new one. This is one reason why there were multiple BBC productions of one-off plays (like "Alice in Wonderland"), as it was less expensive to produce a new version than to negotiate fees for a repeat.
So, with repeats limited, and high fees for re-negotiation, coupled with no home video industry to sell to the public, the UK television companies had these big, bulky 2" tape reels sitting on shelves that they couldn't do anything with. Add in the same reasons US broadcasters wiped stuff (expensive cost of tapes + storage), and you can see why they did it.
As for material that was sold overseas (that was transferred to 16mm prints by BBC Enterprises, not the archives), that material was routinely junked when contracts with writers and actors for international sale expired. Since broadcasters worldwide were abandoning black-and-white for colour, it wasn't seen as worthwhile to negotiate extending these agreements, as there just wasn't as much demand for black-and-white.
Even when home video became a reality around 1976, home cassettes were expensive, and most people paying that kind of money for a tape spent it on movies. There weren't really a whole lot of TV show home videos before the 1980s, and by that time, TV companies had (mostly) stopped junking their archived programming.
So, in hindsight, a really dumb decision, but if you think of the time in which the decision was made, it's really easy to see why they did it.