Having thought about this a bit and read most of the posts, have thought of another option. (All of this because there is no party supporting women's sex based rights).
If there is a party whose main politcies you agree with maybe vote for them, and should the candidate get in email or even write and say you voted but absolutely do not support their position on the GRA and that they should be aware you do not endorse them and will be monitoring their participation in the GRA discussions, changes.
We could set up a GRA monitoring group. List all the new MPs who got a vote despite their parties lack of support for women's rights, and then list whether they even bother to participate or vote on the issue in the next Parliament.
This keeps it a live issue as I am sure the next few months / years will be Brexit grandstanding.
My potential MP has stated she is personally committed to ensure women's rights as under the EA are maintained but as her Party (Labour) isn't, not sure what to do. But may contact her after election as I am assuming she will get in as it is a safe seat.