It is.
But all this is further muddied by the exact wording of the Equality Act (which might well be problematic in the Supreme Court ruling). This says
Gender reassignment
(1)A person has the protected characteristic of gender reassignment if the person is proposing to undergo, is undergoing or has undergone a process (or part of a process) for the purpose of reassigning the person's sex by changing physiological or other attributes of sex.
(2)A reference to a transsexual person is a reference to a person who has the protected characteristic of gender reassignment.
(3)In relation to the protected characteristic of gender reassignment—
(a)a reference to a person who has a particular protected characteristic is a reference to a transsexual person;
(b)a reference to persons who share a protected characteristic is a reference to transsexual persons.
This automatically lumps Self ID into the same protected characteristic as anyone who has a GRC. So I do hope the 'how long is a piece of string' problem is also addressed by the SC.
In theory atm you can be thinking about transitioning for half an hour today and during that half an hour have different rights to the half an hour either side. Potentially including legally being biologically female even though you have a penis. But not have to prove to anyone else outside your head that you were the opposite sex during that half an hour. Everyone just has to accept your word for it.