I am looking forward to this.
Any definition of sex will have to comply with the GRA and the EqA, in particular this phrase, which acknowledges that sex can change. So, either you can change sex, or sex doesn't mean chromosomes - its definition is broader and context-dependent, as the law suggests.
You choose.
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.