Annoying though it is, it is possible that it is impossible to exclude transwomen from the Ladies' pond despite the single sex exemptions in the EA. The argument for exclusion is that it is a proportionate means of achieving a legitimate aim.
Ordinarily, the privacy and dignity of women would be a legitimate aim and exclusion of transwomen is a proportionate means as there is a mixed sex pond. The fly in the ointment is topless sunbathing. Women, in order to preserve their privacy and dignity, want to sunbathe topless away from male eyes.
However, there are a small number of transwomen who have enlarged breasts as a result of medical intervention. They could argue that exclusion would either remove their access to topless sunbathing or deprive them of their privacy and dignity by forcing them to expose their breasts to the possibly prurient gaze of other men.
You would hope that any judge would reject this argument but there is no guarantee.