Its irrelevant if he has a GRC or not. Your workplace is obligated to provide toilets, showers and changing rooms that are either single sex or single use. Its not optional.
It a man is in the women's shower at work, he is harassing the women who want to have a shower, and the workplace should back up its female employees.
Both the Gender Recognition Act and The Equality Act list instances where it is lawful to exclude men from a women's service, even if he has a GRC. Its accepted that men don't belong everywhere. They don't belong in a breastfeeding room. They can't claim benefits intended for pregnant women. We aren't seeing them take claims for those benefits to court because they are clearly laid out in the legislation, so they can't win.
Activists are pushing at the places where they think they can get away with it. They aren't acting for the benefit of women.