What's the evidence again that the majority of UK cinemas projected at 1.66:1 regardless of recommendation, and that they did this right through into the late 70s? I must have missed it.
Even if it is the case though, I don't think it precludes 1.75:1 being the composed for ratio. It is after all, mentioned as well. The difference is so small* that I doubt the film makers would have been beside themselves to see their compositions receive a tad bit more space. Don't forget that many of these films were being shipped over to the US as well, where 1.85:1 was completely set in stone. Better to compose at 1.75:1 and lose a sliver of image in American cinemas, than to compose at 1.66:1 and lose a fair sized chunk, even if it means UK cinemas showing too much.
*Though not always small enough. On the Blu-ray for The Mummy's Shroud, which is of course 1.66:1, there is a shot which establishes the location of the Mummy in the museum he is being kept in (in this one, he's standing rather than in laying in a case), and then there is a quick cut to a close up of his upper body. Despite the previous shot showing that no one else is in the museum, the close up shot reveals the top of a fez at the very bottom of the frame, showing that there is an actor stood in front of the Mummy, even though no character is (it's later revealed that this shot has been duplicated from a different scene where actors wearing fezes are indeed stood beneath the Mummy). In 1.75:1/1.85:1, that "movie mistake" is not visible.
In addition to that, you also get every mid shot and close up having ample headroom to make 1.75:1/1.85:1 work too.
Interestingly, the 1959 The Mummy admittedly looks spot on at 1.66:1, with heads always at the very top of the frame in mid shots and close ups. It could be missing side information, it could just be a case of cropped heads at 1.75:1 being intentional, as Bruce has suggested. But I'd really like to see what it was recommended at in Kine Weekly, if anyone can place hands on a copy.