That there are two sexes, and that people cannot literally change sex, are plain statements of fact that also align with British law. Sex is recognised as a matter of biology. Since 2004 the Gender Recognition Act has enabled people to change their legally deemed sex for certain purposes, but it also recognises that this doesn’t change their actual sex (for example in relation to motherhood or fatherhood, sport, religion and inheritance).
Matt Ridley could get a gender recognition certificate or whatever it is and he’d still be Viscount Ridley. His older sister (no idea whether he has one) could not have got such a certificate and become Viscount Ridley.
That is, we’re back where we should be. Conflating sex and gender normally doesn’t matter, changing also doesn’t normally. But sometimes it all does. So, the Big Q is when, isn’t it?