It's good to see the WRN letter references not just the EA, but also Workplace regs - provision of single-sex facilities is not just an EA thing, as a lot of the negative reaction to the SC ruling has suggested: workplace, health and safety and building regs all have something to say about real-life rather than theoretical women-only spaces.
The fact that most existing buildings already have sex-segregated toilets, for instance, means that providing an adequate number of women-only toilets is usually a 'no change' situation, unless an organisation decides to remove some of them, as the Barbican did.
And building regs also require sex-segregated facilities in the first place, with 'universal' cubicles as an option in some circumstances, e.g. lack of space, in new builds.
So it's not like the provision of women-only toilets involves any great expense or disruption, they are usually already there.
It's the addition of gender-neutral toilets for the small number of people who will not use the toilet designated for their sex that causes great expense and disruption.