'Third spaces' [actually fourth - the disabled toilet is the third space and is NOT for able-bodied trans people to 'feel more comfortable' in] and unisex toilets may be a useful idea but they are not the solution in the short term.
Separate single sex toilets should be provided unless there isn't enough space, in which case a building-regs-compliant unisex toilet suffices.
If there is extra space, a unisex toilet toilet may be provided in addition to single sex toilets.
As PPs have said, building regs have specific requirements for 'universal' toilets, you can't just slap a unisex label on existing women's and men's toilets.
So building-regs-compliant unisex toilets would have to be added to existing buildings, if there is space for them, while leaving the men's and women's as they are, as single sex spaces.
It's not a quick fix, it would take time, and a lot of money, and there's not a lot of that floating around at the moment, is there?
And no, able-bodied trans people may NOT use the accessible toilet in the meantime, the disabled toilet is for people who need it, not for people who choose not to use the toilet designated for their sex.
As far as new buildings are concerned, there should be enough room provided in the plans for the required separate women's and men's toilets, and unisex ones. So the suggestion that going forward all buildings will only have unisex toilets is not right, building regs say they have to have separate toilets - unless it's a tiny building with only space for a universal toilet.
There's no evidence that transwomen are at risk in the men's toilet, so there no actual need for third spaces, unisex toilets or appropriating the accessible toilet.
The simplest solution is for men use the men's toilet, women use the women's toilet, and disabled people (only) use the disabled toilet.