See, I think we should be able to force Primesight (a secular business, without any religious affiliation) to put the posters up for the very reasons of preventing arbitrary or insidious mass silencing. If the statement/ slogan is not defamatory, illegal, untrue (eg TWAW) etc, with strict exceptions for the genuinely religious.
If the bakers had been asked to cater the wedding wearing T shirts expressing that belief, or asked to greet guests with that statement, or wear a sandwich board, that would be different IMO as it would be a very personal association.
Icing or printing words is not forcing the same level of personal association, or the same as actual speech, but if they contravene the rules of their religion there is a case too, otherwise I think not, as mass silencing is a greater risk.
What would happen if the Metro had refused the Women's Place advert? And if/when twitter and all GC women?
It is the global/ national scope of businesses nowadays that I think makes a difference to this. We can see that as women we have so little power, even as consumers, and I don't think this helps.