The answer depends on who you are asking!
Stonewall say the ladies facilities are for anyone with inner lady feelz (and those of us who have female bodies and no particular inner feelz should be told to change in a cupboard if we don’t want to share with Anne)
The guidance on the Equality Act 2010 says that the ladies is for biological females and the (vanishingly) small number of transsexuals with a gender recognition certificate are who are indistinguishable from the sex they have transitioned to look like.
A huge number of public services and businesses have been trained by Stonewall (and Mermaids et al) to believe that the first scenario is the law.