I'm trying to get my head round what would need to be covered in legislation if the law accepts the fact that sex is not gender and that gender doesn't alter sex:
Sex is stable, so should be what is recorded in official documents and databases. It shouldn't be altered, and ones that have been altered/ misrecorded should be changed back.
If gender is also recorded, it should be an additional, optional and amendable field, which would allow for detransition.
Invasion and appropriation of opposite sex spaces and sex-related language etc should be off the table.
Children and young people should be protected from life-changing medical and surgical interventions until they are of age. This should include being allowed to explore all options therapeutically with a patient without being accused of "conversion therapy"
Nobody should be penalised, in law, or under workplace rules etc, for recognising sex as a reality.
With those caveats, there should be recognition that some people need/want to live and present unconventionally for their sex. Whether this is best defined as a natural condition, or as a belief, they would need the following:
Protection from discrimination (in employment, housing etc) relative to other members of their sex/ members of the general public, and from harassment, arising from how they appear, dress and present. There's nothing unique about that; it is analogous to protecting anyone else who "looks different" e.g. because of race or disability, or who wears special clothing because of their beliefs e.g. hijab, turban.
(As with religious dress, there will be negotiations to be made in specific instances where requirements conflict with the requirements e.g. of health and safety or a workplace dress code. This would cover things like turning up to work in inappropriately suggestive clothing.)
The following would probably depend on how gender is legally defined (a condition? A belief? A choice?):
There would be a discussion to have about what kind of gender related treatments should and shouldn't be a) legal at all and b) available on the NHS.
Providing separate spaces for people uncomfortable or not easily accommodated in spaces for their sex - is this most similar to disabled "reasonable adjustments", or to providing time out and a prayer space for Moslems, or what?
What other things might need to be covered?