Disabled toilets should be available to all peeps but with priority if you are disabled. What if you have a need to remove all of your clothes just to have a wee. Maybe you'd need just a bit more room than normal. Maybe you'd need two people in there tto get you ready. There are countless reasons why large/disabled toilets are needed. 

Should they be locked? That's a different kettle of fish altogether. I knew one Bingo hall that had the toilets behind the he main are where you bought your Bingo books. Ladies on one side and men on the other. Each side had a disabled toilet section in it and it was kept locked at all times. The key for the door was kept held in place where all could see it. If it was there, you could take it and go about your business. If not, you could maybe try the one on he other side.

It seemd to work ok.

Cheers,

Steve.