Case by case basis is what we already have in society now. It's just that the service providers are failing to conform to the law.
The bottom line is that if it is a single sex provision, no members of the opposite sex can be admitted. If members of the opposite sex are permitted, then it cannot be a single sex provision.
It just needs service providers to stick to the law or be sued - as in the Darlington case. And with clarity from the SC, the EHRC, the judgement in the GLP case and the government, it will be a slam dunk and no one will want to be taken to court over failing to provide SSS.
There can be no other outcome. The judge is not going to agree with the GLP that some men are women, even though there is no way to define at what point they become women and that admitting those men would not be discriminatory to other men. There is no way the GLP are going to get a ruling in their favour because what they are asking for has no legal standing, is discriminatory and does not even make sense.