Hi Kate
The very first prosecution report that I wrote as an HSE Inspector included charges for inadequate welfare arrangements.
However, to be honest, if the toilets and washing facilities had been fit for purpose I wouldn't have been particularly bothered about which sex was to use them.
....and perhaps strangely, nobody suggested that as a male HSE Inspector that I shouldn't be looking at what the facilities were like when they were clearly designated for use by women only. Someone from management would check that a woman's dignity was not about to be compromised but nothing in Section 20 of the HSWA restricted my powers of entry to NOT cover facilities for the other sex.
Then when I moved out of HSE and it came to recruiting there came the time when we were looking to add to our team. One structural engineer asked me a question:
"Have you found someone yet?"
"Yes"
"What's his name?" - this person should probably have known better.
"HER name if X and HER name if Y".
"But they will have to inspect constuction sites?"
"Yes, what's the problem?"
"Well sometimes the toilets are, you know, a bit bog standard."
"Well they shouldn't be and if a woman finds them like that, perhaps the Contractor will be even more embarrassed than if I tell them this."
I doubt that this Supreme Court ruling is going to cause many problems except for organisations that can't keep up with the times.
There will be a few women objecting to a transwoman using their facilities (and vice versa for the men) and employers can deal with that, if necessary asking the trans people to use the disabled loos.
However, if the trans person has been integrated into the workforce, I imagine that the complaints will be limited, particularly once things settle down.