I have had an email today from the Equality and Human Rights Commission which will please nobody. Their understanding of sex in the Equality Act 2010 is that it means sex on a person's birth certificate, either issued at birth or under the Gender Recognition Act.
That's narrower than trans people would want (and I still think they haven't properly reflected the latest MB judgment in the ECJ) but it also confirms that it is not the biological definition gender critical feminists want so excluding someone with a GRC will be extremely difficult.
They say that they are in the process of formulating new definitions of sex, gender and gender reassignment.