Here's my email, I'm sharing it because this was actually my experience this weekend and so I wanted to prove that it's not just middle aged women getting arsey, it really does make a difference:
I know you've had many similar emails, but I wanted to add my voice.
I will be boycotting M&S from now on because of your changing room policy. This is actually a massive pain in the backside for me, because my nearest food shop is the M&S Simply Food, and we usually pop in at least once a day for newspapers and top up shopping. We are now using the Co-op and Lidl.
Why do I feel strongly about this? Well I took my thirteen year old daughter for a bra fitting yesterday. She's a 30F, and embarrassed about her figure. She scarcely wanted to go anyway, and would never have stayed for five minutes in a changing room with men next door. So instead I took her to Bravissimo, where she felt safe and secure, and she could take 45 minutes. We spent £120 in there, but it was worth every extra penny of expense for the experience.
I'm not a transphobe. What I am scared of are all the other men who are going to take advantage of the policy in order to get their thrills, which mostly come from undermining women. There is no way of stopping this until someone is assaulted, no way to get rid of the hidden cameras until the are put in, no way of assuaging the nerves of embarrassed teenagers until they have been humiliated. Please can you tell me why the needs and comfort of a small number of trans men are seen to outweigh the needs of women? It's perfectly possible for the store to satisfy both, but it's just that you don't want to.