In all my workplaces (mainly central London, very large organisations) the standard layout was Men's plus Women's plus a single Unisex One-Room toilet large enough for a Wheelchair/Guide Dog to turn round in.
There was no official guidance for who used the Unisex. Typically it was very much underused as there was only 1 or 2 people with a wheelchair or dog per floor at most. Often it was zero.
So the Unisex was mainly left unused, just provided for a very occasional overflow, or to deal with a period/coffee incident, or to change in, or possibly have a quiet moment alone.
I understand the approach now in those large organisations is that trans women are being told to use the Unisex. They could use the men's but this suggestion is met with fury, so HR don't even suggest this anymore. No-one has carried out any dirty protests that I know of in response to this change. Well-paid(ish) secure white-collar employment is not to be cast aside on a whim, it seems.
If I used a wheelchair or a support dog I would not like this new policy at all. But, I'd have to admit that the current provision was already close to 1 toilet:1-2 employees. So I would probably have to tolerate it, but be very ready to feedback to HR if there were any issues.
It's definitely worth doing a trial. The cost of a 4th space toilet refit would be prohibitive to UK Business (and to our taxes for Public Sector buildings), and almost certainly unnecessary.