The Scottish Prison Service have operated for 11 years on the basis of self-ID. The English and Welsh prison service operated a fairly similar policy, I think, until Dominic Raab in the last government said that men would only be allowed in the female estate if they had had sex reassignment surgery and they were not in prison for sex offences. That should have reduced the number of men in women's prisons significantly.
But as PPs have said, after the Supreme Court ruling there should be no men in women's prisons. They don't need to wait for the EHRC guidance because the ruling was clear and unequivocal. Sex means biological sex. Women means biological women.