This is one area I don't think would be covered by the Equality Act exemptions tbh.
You'd have to demonstrate a need for women to bake cakes and make jam without the presence of males. I think you would potentially struggle.
You would be able to prevent transwomen from going to a WI residential but not necessarily from other social groups unless they could demonstrate a legitimate aim from being single sex.
So a kayaking session for women because of the difference in levels of physical ability and how women are deterred from participating in sport in the presence of stronger males who have been socialised differently MIGHT just get away with it but I am struggling to think of a specific legitimate aim the WI are trying to achieve that would be deemed proportionate in a court.