I am 38 weeks pregnant and I have SPD. I can walk about 20 metres, but its agony. This is a temporary condition that will hopefully disappear in two weeks time. I am not registered disabled or consider myself to be disabled. At worst I am in a similar position to someone with a broken leg.
I went to a cafe today and I used the disabled toilet because I could not face the pain of climbing up the stairs. There was a disabled person waiting to use the loo when I came out and I hope they didn't mind too much.
I would not take the piss use a disabled loo under normal circumstances.