So, I wrote to M&S and part of their response to me was to tell me that, even though I'd explicitly said that both I and my daughters feel excluded by their new policy, I wasn't really excluded and they were doing all this in the name of 'inclusivity' (how ironic). Gaslighting, basically.
They also told me that all their changing rooms are lockable and have mirrors so I really shouldn't worry (what a silly woman I am, pat on the head time). Of course, if I shouldn't worry for this reason, I do wonder why transpeople have a problem in the changing room of their natal sex. I did point out this double standard.
I also suggested a third unisex / gender neutral space as a better solution that includes all. They said they'd think about that.
What I didn't do, and I wish now I had, is state that actually one of my biggest problems with their new policy (and GG and YHA) is that they are lying to the general public. It's fine if they now want unisex changing rooms, or 'feminine' and 'masculine' changing rooms (which I feel is what it is really, gender stereotypes but unisex). But be honest. Because I know my daughter if she went into a changing room (or toilet) labelled 'women' and encountered a man would be scared and upset. Because she'd been lied to and misled (that's what she'd think).
Most people think that 'womens' and 'mens' means segregation by sex, not gender. So when they change that and are not clear and honest about it, it's a problem. And I do feel I'm trying to prepare my daughter for the world and it's made even harder by this sort of shit. Because it is lying.
Anyway, given how I was condescended to by their response I have no more time for M&S and will remove my financial buying power from their stores which I suspect will have a lot more impact than my words.