If sex is defined, in law, as something immutable (which is what this amendment proposes) then gender reassignment is no longer possible.
The current situation seems to be that sex in law is defined as "sex at birth. potentially modified by a GRC".
So if you think "gender reassignment" is governed by the legal definition of "sex", then you believe only people with a GRC are currently covered by "gender reassignment"?
That's a remarkably narrow position, and not one I think many hold. The usual view is that any attempt to pretend to be the opposite sex works, it doesn't have to be a legal fiction.
Do bear in mind that we are planning to abolish GRCs, and I think we were broadly okay with the "gender reassignment" EHRC protection remaining, but if you want to tie that to GRCs so it vanishes too, fine?