The 2004 law doesn’t make much sense, but removing all criteria means that effectively sex based rights would be replaced by gender based rights.
And it means that anyone can identify as a woman (or a man), by filling out a form.
You can change legal sex and have an F on your birth certificate.
And yes, women can do the same in terms of having M. But there are no women clamouring to get into male prisons/changing rooms/sport.
(Which is how we have ended up with convicted male born rapists being sent to female prisons and assaulting the inmates).
^ in answer to Mammajay