Single sex exemptions can include GRC holders. That is the point of them. The proportionality for application is higher for GRC holders than not, that's all.
Ok, so we agree. Whether someone has a GRC or not is important. A man who identifies as a woman but doesn't have a GRC is more easily excluded than one who does, since the legal sex of the former is 'male' and the latter is 'female'.
If they bring in self-ID alongside better enforcement of the single sex exemptions, there will be many more men who are legally female, making it harder to exclude them from female spaces.
For example the new prison policy I referred to earlier. A man who has a GRC making him legally female is automatically initially placed in a female prison. Without a GRC he is legally male and goes to the male prison and has to go through a process if he wants to be transferred.
I have no idea whether the prison policy is compliant with the EA2010 (I think it is, but IANAL), but it's what is in force now. The EA2010 appears to make it possible to make a judgement of when people with the PC of gender reassignment should be excluded on a case by case basis. The prison service and MoJ seem to have decided that legal sex is more important than biological sex. If RadFemLawyer thinks that they are acting illegally maybe she could tell them.