To clear up a few misconceptions...
1. The episodes weren't destroyed because there wasn't space to store them; the UK co-producer, TVS, lost its franchise in the early 90s and closed down. Its archive was sold off, and after a few changes in ownership eventually passed to Disney. Unfortunately at some point the paperwork was lost, so there are two problems: noone knows exactly what is on the tapes; and they will be very difficult to clear for DVD release because noone knows who needs to be paid royalties.
It is probable that at least some of the archive was destroyed when TVS closed. I have an eyewitness account of tapes being thrown out, although at this point it isn't known what was on the tapes. Whatever remains was sent to Disney in the US some time last year, but we still don't know what was in there.
2. The Zimbabwe thing is an in-joke on that forum - it comes from a particularly inane story posted in The Sun (for non-UK readers, it's a tabloid paper) which claimed that Robert Mugabe was hoarding lost Doctor Who episodes...! The poster who made the comment was not in any way being serious. The earlier poster who claimed to know where the missing tapes are has not been seen before or since, which pretty much makes him a troll. There is no reason to believe that he knows anything whatsoever.
3. The site with the list of surviving materials is mine
The list is accurate to the best of my knowledge, but if anyone does have information - or better still, recordings - not shown there then I would be grateful if you could drop me a line. I'm coordinating the hunt for the lost episodes.
4. The clips from six UK episodes uploaded to Henson's YouTube channel this summer most probably come from a set of tapes held at The Paley Center in New York. These tapes are Betamax (and hence not broadcast-quality), and are NTSC conversions of the shows, not the PAL originals. It isn't known where the recordings came from. The reason we think the YouTube clips come from those tapes (and not from some other source) is that firstly the YT clips are NTSC; and secondly the only six episodes held by The Paley Center are the same six that Henson uploaded. PAL copies of all six exist, so these tapes are of no interest.
5. There is no such thing as 'grainy VHS'. Grain is a film artefact and has nothing to do with videotape. Of the 57 episodes confirmed as existing, 29 are broadcast-quality (12 as released on DVD, 17 at the BFI), 12 are commercial VHS releases from the 80s and the remainder are off-air VHS recordings.
6. With the exception of the 12 DVD-released episodes, which Henson have, and the six at The Paley Center, there is no reason to suppose that the episodes ever left the UK. The show was a UK-specific production; other countries either had their own (France, Germany) or took the US show. It is therefore vanishingly unlikely that any of the tapes will turn up outside the UK.
7. ITV are scheduled to air an as yet unidentified episode of Fraggle Rock on Sunday 6th January. We do not know whether this will be a UK episode or a US one.