noblegiraffe
It's no surprise that you're confused. Because there are conflicting bits of information in the different pieces of legislation. And a lot of it is open to interpretation, until caselaw is established.
That's why the term Stonewall Law has been coined.
The EHRC were forced to confirm that without a GRC the comparator over whether you are being discriminated against according to sex is someone of your own legal sex. If you have a GRC, your legal sex will be opposite to your biological sex. And your legal sex is now the comparator (but, interestingly, there are still some exemptions built in, the rule of primogeniture, for instance).
So without a GRC, a transwoman's comparator would be other men.
Children can't get a GRC until they are 18.
The protected characteristic 'Gender reassignment', as far as I know, does not specify an age. And the guidance is someone should be treated as their preferred gender.
However, as exemptions have been built in, it's clear that exceptions can be made. And the term is if they are 'a proportionate means to a legitimate aim.'
It's absolutely acknowledged that places like rape refugees and sport should be subjected to those exemptions. But using a phrase like a proportionate means to legitimate aim is rather woolly. And, in which case is open to opinion and interpretation.
Providing a schoolchild with a separate, perfectly acceptable toilet would, to most people, constitute a proportionate means. And the legitimate aim would be so that the other children (girls usually) maintain their privacy and dignity in their own toilet.
Again, as far as I know, case-by-case is not actually written anywhere. And you can have a blanket policy. Going case-by-case would be a nightmare of why you would let one person in and not another.
This is my understanding, but because it's so confusing and often contradictory, I'm happy to be corrected.