I watched the reconstruction of "Marco Polo" today, and a thought suddenly occurred to me, a question I hope someone here can answer. I know that in the early episodes, material such as special effects, fight scenes, and location scenes were shot on film a few weeks ahead (usually at Ealing Studios, except for the location stuff), then played back at the appropriate moments in the studio shooting sessions. So that means that portions of these episodes existed on film separately from (and before the creation of) the videotape masters.
Now, I know the story of why the finished episodes were erased, but these pre-filmed inserts were on physically separate media from those final episodes. So, given that most or all of the episodes would've had at least some pre-filmed material, you'd think that at least portions of them would still exist in most cases. (Sort of like what actually happened with "Shada" -- all the scenes filmed on location exist because they were all shot first, before the studio portion that was unfinished.)
But as far as I know, none of these pre-filmed sequences have survived. When an episode was wiped, everything was wiped, except for copies sold overseas or sometimes censorship trims that were cut out of those film copies. So why would the original filmed insert scenes have been destroyed? If there are surviving censorship trims, of all things, why wouldn't this original production material have survived? I mean, surely some of the filmed effects might've been worth keeping around as stock footage, if nothing else. So what happened to it all? Even if the BBC hadn't felt a need to preserve it, you'd think at least some of it would've survived piecemeal. So if it's all gone, that implies a systematic policy of destroying it. But why? They were on film, so there's no way to erase and reuse it like with videotape. So what purpose would there have been for destroying them? And if they weren't systematically destroyed, then where the heck are they?