Parking in a disabled space when a car park is full

[Second Best]

Today, after work, I visited my local express. It's got a small car park with about 12 spaces, plus 2 disabled. When I visited, the car park was full and the only spaces available were the 2 disabled ones. I did a lap of the car park to see if anybody was leaving, but no luck. I noticed an employee chatting to a security guard not far from the disabled spaces, so I parked in one, got out of my car and asked if I could actually park in one seeing as there weren't any other spaces available. I was told no, it's "illegal" and to find somewhere else to park or I'd get a ticket.

I did another lap of the car park, no joy, so went to the local Waitrose which had a more reasonably sized car park. No big deal, and in fairness Waitrose were doing a surprisingly cheap deal on what I wanted (12 Heinekens for £8.80!) so it was a blessing in disguise. However it got me thinking as I was driving home - are disabled spaces only usable by disabled folks, even if the other spaces are full?

Just to explain, I have no problem with disabled spaces and would never park in one out of sheer laziness, but is the expectation that if no generic spaces are available, then the car park is only for disabled people?