Hannah Pearson, of the Equality Network, backed the decision to ban the signs from buses and argued it was the “vehement anti-trans” campaign behind them, rather then the definition of women itself, which caused offence.
The recent decision with the bakery and their right to be compelled to say something they disagree with seems relevant here. In that case, the bakery won because they were happy to sell a cake to a gay couple, but didn't want to be compelled to write a message they disagreed with on it. i.e. they were willing to provide the same service to customers, but not to be compelled to say something they didn't believe.
What the advertising company seems to be saying is the reverse of this - that they are perfectly happy with the wording, but that they don't want to be providing a service to a particular group. Does any legal-type person know whether this would therefore constitute discrimination?