Parking rules/regulations?

[twocultures]

Just out of curiosity.
Me my DP and DS (11months) went to a big shopping mall over the weekend, it was packed, we struggled for a parking space so my DP suggested he will drop us off at the entrance (drop off/collection point) and get the pram and baby gear out of the car so me and DS can get sorted whilst he looks for a parking spot.
As he set off, in front of him to his left a car pulled out of a parent and child spot but before he could do anything a woman going in must've noticed the spot opening up, turned into the lane even though there was a clear no entrance sign as the traffic went in the opposite direction to her and despite my DP indicating that he was waiting for the spot she indicated too and cut him off. Even though my DP was there first.
I saw that it was a young couple, really athletic in their early 20s but he had a cast around his ankle (up to mid calf) and used crutches.
My DP had to look for another 20mins and ended up having to park far away with several flights of stairs along the way which was a bit of a bugger to get the pram down when we were returning to the car.

We didn't mind really as obviously he had a cast on but it just got me thinking so I wander if anyone knows if there is any and if so what are the laws/rules or regulations regarding parking in parent and child spots? Who can and can't park there etc?