"It's based on a discretion to include certain males in a female space, which is not possible under EA Schedule 3 para 27, but is arguably possible under the WR."
It sounds as though you may be alluding to regulation 24(2).
(2) Without prejudice to the generality of paragraph (1), the facilities mentioned in that paragraph shall not be suitable unless they include separate facilities for, or separate use of facilities by, men and women where necessary for reasons of propriety and the facilities are easily accessible, of sufficient capacity and provided with seating.
[emphasis added]
So there must either be separate changing facilities for men and women or, if there is not, then it is permissible for the same facilities to be used at different times by men and women.
So, for example, if there were a notice that said something like "Men only from 7.30-8.00 and Women only from 8.00-8.30" then that likely will be lawful. (EDIT - although there may still be a case for indirect discrimination)
But that is not what was happening in Fife.