I had a good chat with a lingerie assistant in my local M&S branch today. She told me that they were vigilant in the lingerie section - no men allowed in - because of all the responses spelt out here (first bras, mastectomies etc.) All good until a few minutes later she admitted that if the person was dressed as a woman, then they had to be allowed in, because otherwise, they were told, it was discriminatory.
Same as their former trans colleague using the women's staff changing room; the female staff didn't like it but were basically told they were being discriminatory if they complained.
I told her bluntly that they were still men, probably with all their bits intact, and she agreed it seemed wrong, but there was nothing she or her shop floor colleagues could do. Senior management ignore them too.
I mentioned men accessing the children's fitting room. She said there was probably some rule about adults entering without children (say under 16) but she didn't really know. She mentioned safeguarding, so I'd like to think I've raised awareness in the store today at least
I also went into a fairly new John Lewis. Curtains on the cubicles, but you could easily stand on the stool and discreetly position a phone over the top (and I'm only 5'1"!) But at least without the lockable doors, they can perhaps have a legitimate single-sex policy.
Fitting rooms in JL say just that - no mention of men's or women's - but I did noticed there was one that specifically said Children's fitting room, so that at least was a positive..