“provided it is proportionate, the female only nature of the service would engage paragraph 27 [of Schedule 3] and would permit the exclusion of all males including males living in the female gender regardless of GRC status.”
"There is nothing in the judgment that suggests that the question of what is “proportionate” is a question to be negotiated on a case-by-case basis with individuals; rather it relates to whether the rule is lawful.
The judgment describes how service providers offering a single-sex service are also exempt from the prohibition against gender-reassignment discrimination (as long as their conduct is proportionate). For example, a “trans man” (a woman living in the male gender) can be lawfully excluded from a female-only service, without this amounting to gender-reassignment discrimination"
https://sex-matters.org/posts/publications/supreme-court-judgment-summary-and-practical-advice/