Yes, thanks, I knew most of this.
Just a few points: It was actually BBC Enterprises in the 1960s and 1970s. TV shows were marketed under conditions but with one important addition; if the prints had exceeded their agreed sales target and weren't to be sent to another TV station, they were to be either sent back to the BBC or destroyed, and a certificate of destruction issued as proof. I suspect that many episodes were routinely destroyed as part of the sales agreement; it is cheaper to burn or thrown out than go to the hassle of shipping them back.
You're right about the Film Archive though. Only a random assortment of 16 and 35mm black and white episodes wound up there. I forget how many, possibly about 30 or so. Then when it was found that BBC Enterprises had made telerecordings/kinescopes of practically all the episodes, efforts were made to find out what they had. Unfortunately, they had been destroying episodes for about 5 years at that point so there were big gaps. The BBC Film Archives stopped the junking almost straight away and requisitioned the exisitng film prints from Ents. Later on, VT was also requisitioned, and it became the BBC Film and Videotape Library. By the time this had happened, a lot of colour video tape had already been wiped by the BBC Engineering Dept. for reuse.
One thing we must bear in mind is that this didn't just affect Dr.Who. Nearly all of the BBC output was affected. Some dramas have many hundreds of episodes missing. And it isn't just a BBC problem. Many Independant TV station in the UK have lost material. Its just that Dr.Who is soooooo high profile ;-)