Relevant section from Supreme Court judgement (reported very badly in the press/social media)
While many women in a female-only changing room or on a women-only hospital ward or in a rape counselling group might reasonably object to the presence of biological males, it is difficult to see how the reasonableness of such an objection could be founded on possession or lack of a certificate. This is so especially when the distinction does not track physical appearance or presentation, and the woman is unlikely to have any information about the GRC at the point at which her objection might be raised. A trans woman with a GRC who presents fully as a woman may feel she is more likely to prompt objections from other users if she enters the men’s changing room or other facilities than if she uses the women’s changing room or facilities. But in facing that dilemma she is in the same position as a trans woman without a GRC. Although such trans women may in practice choose to use female-only facilities in a way which does not in fact compromise the privacy and dignity of the other women users, the Scottish Ministers do not suggest that a trans woman without a GRC is legally entitled to do so.
It's clear from photos that have been circulating that Dr. Upton was discernibly biologically male at the time and so Dr. Peggies objection was reasonable. He also acknowledged he was frequently "misgendered". In the Darlington nurses case, the individual was sitting in boxers with telltale bulges. In both these cases management could have made a space for an individual trans changing facility.
In other cases though I think it's unnecessary and not practical. I went to ballet classes where there was a womens, men's and some sort of gender neutral changing, split from previously being two. Each room had one private cubicle and an open changing space. Most were biological women who couldn't fit in "our" room! Any trans woman would have been fine in the men's or using the loo. Ballet classes aren't exactly dangerous for non gender conforming people, and if anything this creates resentment against trans folk who as far as am aware never asked for this anyway.
Personally I don't have an issue with fairly female looking trans women probably on hormones using public loos in places like busy shops. As long as they are discrete. But it has to be in a way biological women feel comfortable with and are safe. Disabled loos need to be available as far as possible for disabled people who may have urgency needs / stomas leaking etc. Someone who is trans and their physical appearance is of that sex should be using facilities for that sex.
Sorry for the long post. Hope it is of interest.