It's the insidious and secretive nature that is the problem.
At least be HONEST about what you are doing M&S!
Put the proper MIXED SEX signage on all the doors and actually let people know that they will be sharing their changing rooms with men.
If you are so confident that you are doing the right thing, then you would have no problem doing this, and letting the (I think) middle - older woman demographic who are your main customer base, KNOW your policy, most of whom I strongly suspect would not want to share these spaces.
But you wont do that, because you know it fucking stinks to high heaven and you want them to still spend their hard earned money with you, while fucking over their right to privacy, dignity and safety.