I'm not sure what shop I'm thinking of, could be Hobbs. Where they have individual changing rooms towards the end of the shop, but basically they open out onto the shopfloor.
They may have changed it, but they didn't used to have mirrors in. The only mirror was the communal mirror on the shopfloor.
So without knowing what on earth you looked like, you would have to go outside to see.
I don't know what the training was, but standing in the middle of a public place positively invites comment. Largely from the assistants, but also from random members of the public. Even the waiting male partners of some of the women.
I would only ever shop there as a last resort and my heart would sink as I entered, with an almost subliminal scan of who was sitting on the waiting chairs.
I'm a fairly body confident woman. But, as I pointed out above, difficult to dress. The process can be tortuous.
Privacy makes a massive difference.
I'd like to think women will vote with their feet. But I don't think they will. Most will just go on being uncomfortable.