There might be a standard 'standard' that they should all meet, but in reality disability is often so complex that a generically 'accessible' loo rare, and every loo is different (and that's without considering that many technically accessible loos that are used as storage rooms, rendering them inaccessible.)
For example, some people need space to the right of the loo to transfer from a wheelchair, some to the left.
Some people need adult sized changing mats, others need a toilet, and others need to empty appliances.
Some people need grab rails within easy reach of the loo - while others need free space around the loo so their chair can get close enough.
Trying to fit all this into one bathroom is challenging, but it's doable.
My favourite loo does an amazing job of covering a wide range of needs (although I'm not sure how well it would suit someone with a visual impairment.)
It is at Paddington Railway Station, near platform 13.
An adjustable adult changing mat with hoist, and shower.
Seriously, it is SO nice to wash your hands without having to do minor contortions to get your arms up and over the side of the sink but under the taps in a standard sink - while simultaneously trying to get close enough to reach without crushing your legs between the chair and the sink.
It's massive and spacious and clean and marvellous and has everything I need - and many things I might need in the future.
True, the hand soap is strangely distant from the taps - a second one closer to the sink would be great, but it's still the best public bathroom I've ever been in. In the same way that one rarely passes ones favourite coffee shop without popping in, I rarely pass through Paddington without stopping there.
So big thanks to Paddington Station for a fabulously accessible loo.