@BertrandRussell I think it comes from a misunderstanding of the law, which I find quite interesting as a lawyer.
At the moment the definition of a woman is adult human female, plus the GRA 2004 creates the legal fiction that an adult human male with a Gender Recognition Certificate is female.
The EA 2010 then goes on to say that there must be no discrimination, harassment or victimisation of anybody on the grounds of gender reassignment, including those who are proposing to have it or in early stages rather than those already in possession of a GRC. Sensible enough, and that also protects those like me who are sometimes mistaken for a trans man (I'm not).
EA also says that someone undergoing gender reassignment should be treated as a member of their preferred sex unless the provision of a single sex service is in proportionate pursuit of a legitimate aim (s.28 to Schedule 2). It does not as far as I can see say that that person IS a member of their preferred sex (unless they have a GRC in which case the GRA 2004 kicks in).
The accurate legal proposition of "trans women with a GRC are legally female" has been modified to the catchier, but inaccurate, "trans women are women."
I don't think it is impossible to redefine "woman" - after all, looking back over the last 300 years "marriage" did indeed get redefined and in a good way - but it needs Acts of Parliament and judicial oversight, not wishful thinking on Twitter.