A woman who isn't hired for a role because she is recognised to be a female of childbearing age will have no recourse if that employer can demonstrate that he hires other 'women' (who could be biologically male but not recognised as distinguishable from females any longer, since sex is no longer a protected measure)
Unless you have some inside information to the parliamentary drafts office about legislation which is not even at bill stage you have made that bit up.Sex and gender reassignment are both protected characteristics. If gender reassignment is changed to simply gender "gender"
It seems to me that in a situation where an employer has 2 suitable candidates one of whom is female but obviously trans and one who is biologically female an employer will have to find a determining factor other than one being trans or biologically female.
There may be trans women who pass - on the small screen Paris Lees and Blaire Whyte do but do many? Unless "sex" is deleted from the legislation I think your scenario here is fanciful and alarmist which might explain the lack of uptake.
Btw it is "supersede" not supercede. (Sorry it's a pet hate of mine but if you are aiming for an authoritative tone it grates)