Pratchet it is lawful to take positive action (note: not positive discrimination) for the good of any of the protected characteristics under the Equality Act. Not just women. This is allowed where gender reassignment is concerned.
This link explains positive action vs. positive discrimination.
This one describes what positive action is lawful under the Equality Act 2010.
That second one says explicitly:
"It is lawful under s.158 of the Equality Act 2010 for an employer to take action to compensate for disadvantages that it reasonably believes are faced by people who share a particular protected characteristic (ie age, disability, gender reassignment, marriage and civil partnership, pregnancy and maternity, race, religion or belief, sex or sexual orientation)"
Yes, there are specific exemptions written into the GRA which separate transitioned women from natal women in specific circumstances, but the Equality Act doesn't.
That doesn't mean you can't use the equality act to achieve that aim, it just means you have to lawfully discriminate on the ground of sex and on the grounds of gender reassignment. Voila - xx females only - where that is a proportionate means to a legitimate aim, of course.