The thing about The Highlanders missing is, that it's Jamie's introduction story,
The very odd thing about The Highlanders is that it was the very first story to be wiped (NB: that means its transmission tapes were wiped, not that the film copies made for BBC Enterprises were destroyed. That didn't happen till the mid-1970s).
It was wiped in March 1967 within six weeks of its only UK transmission, whereas usually tapes weren't wiped until two years after transmission (when the option of a pre-cleared repeat ran out) - and in practice, every tape was kept until 1967 (including Dalek Masterplan 7, which Enterprises never transferred to film, but which was kept until August '67) as BBC Enterprises weren't happy with the original 'suppressed field' film copies and asked for the tapes to be kept until they'd developed a better technique of making replacements (the 'stored field' method).
As 'stored field' came in at the start of 1967, that explains why Enterprise didn't stop Highlanders being wiped. But it's a bit odd that the producer let it be wiped 20 months early, ruling out any chance of a BBC1 repeat, given that it was the first story for the new companion...