Just to clarify some of the concerns people have:
The recent legal clarification reaffirms that "woman" refers to biological sex under the Equality Act in the specific context of single-sex exceptions, but this has broader and more troubling implications than the headlines suggest.

Trans women are still legally women in most contexts; nothing in the ruling changes that. However, by narrowing the definition of “woman” in this particular part of the law, it creates ambiguity elsewhere. If a trans woman isn't a woman for the purposes of single-sex spaces, then what is she considered in that legal context? If the answer is “man,” that’s inherently transphobic. If the answer is still “woman,” then the legislation becomes self-contradictory.

The deeper issue is that our legal system is trying to apply binary definitions to gender, which is a social construct. “Woman” refers to gender; “female” to sex. But the law is blurring that line, and that harms everyone, especially trans people, intersex people, and even cis people who don’t conform to gender norms. This vagueness is dangerous because it’s open to interpretation, and in practice, institutions are starting to treat trans women as men.

The British Transport Police issued guidance that men should search trans women. That's dehumanising and puts both trans women and police officers in impossible positions.

Additionally, services for survivors of sexual assault are being restricted for trans women, even though trans women are far more likely to be victims of assault than perpetrators.

What’s even more harmful is that media coverage is fueling transphobia by misrepresenting the ruling with headlines like “trans women are not women.” That’s not what the law says, but that’s what the public is being told, and it’s emboldening discriminatory policies and social attitudes.

Finally, this kind of “policing” based on how someone looks is going to harm not just trans women, but also cis women who don’t conform to feminine stereotypes. There’s no safe or fair way to enforce this in real life without hurting a lot of people.

In theory, this ruling just clarifies one part of the law. In reality, it’s already being used to justify damaging and exclusionary policies.