Unless you repeal the current GRA (which changes legal sex for some but not all purposes) and replace it with some sort of ‘gender affirmation certificate’ that has nothing to do with sex, legal or bio and instead gives you solid protections against discrimination in housing, education and law without granting access to anything designated for the opposite sex
You’d need to use completely different words for genders though, not just pretend that male/female man/woman and girl/boy are gender based.(lesbian/homosexual/mother/father etc would also need to be ringfenced as sex based)
Call the gender options transmasculine, transfeminine, nonbinary, whatever. If it’s completely separate to sex and doesn’t use our words it’s not really any of our business (I suppose equalities data would need to collect both sex and gender, with sex being compulsory and n/a option for gender?)
If it’s completely, totally separate to sex then it could probably be done on a Self ID basis?
Young transactivists might actually prefer this but the late transitioning males would burn Parliament to the ground before they’d give up the pretence of being female (see certain middle aged TV presenters offering to show their imaginary cervix as an example of this type of activist)