Maugham (I know, I know) is interpreting it as single sex facilities open to both women and transwomen and excluding other men.
[61] …In a case where the provision of separate lavatories labelled male and female was materially similar in terms of the extent of the provision, location, and so on, I consider there would, in principle, be scope for a strong argument that a rule or practice that permitted trans women to use the “the female” lavatory but required other biological men to use the male lavatory would comprise different but not less favourable treatment on grounds of sex…
This means that service providers may lawfully provide, for example, women’s toilets which both cis and trans women are permitted to use, but from which cis men are still excluded.