If the SO is legally a woman in all ways, as Lady Haldane ruled, then you couldn't discriminate against they for their sex. Because their sex is 'female', legally.
So I guess it would be discriminating against them for their 'gender reassignment', but I don't understand how, if they have become entirely female, they are still covered by that 'gender reassignment' clause. It's a done thing, they no longer have any other sex to have transitioned from.
IA [obviously] NAL.