Equality act says can't discriminate against people based on sex or gender reassignment.
Gender reassignment in eqa is defined so loosely as to mean effectively self ID.
GRC means that legally a person becomes the opposite sex.
Equality act says can't discriminate on basis of sex, but there are exceptions such as sports, sleeping accommodation, proportional response to a need (women's shelters etc). This means that males can be legally excluded from female spaces and vice versa if their is a good reason.
Equality act also says cannot discriminate against gender reassignment. This means that you can't sack someone if they are trans, and you can't exclude a transman from women's spaces or vice versa.
GRC means that someone becomes legally a woman, except where single sex exemptions apply. So can be excluded from opposite (biological sex spaces) but female on documentation, statistics, women's shortlists etc.
Currently pretty much everyone is allowing self ID for everything anyway (prisons, passports...) and pretty much everyone is misunderstanding the law (twaw so cannot be excluded from any female spaces ever)
Self-ID means harder to pull back on all the ahead of the law stuff, and more official stats etc may be wrong. Will reinforce all the ahead of the law stuff. Will reinforce idea that sex can be changed. Will possibly impact ability of children to access medical and surgical transition. Other issues with self ID?
What really needs to happen is repeal of GRA completely, reinforcement of single sex spaces, and society to be accepting of people who don't conform to stereotypical gender stereotypes?
Sorry for the wall of text, but I'm am trying to get it straight in my head for the next time I get doorstepped by a canvasser!