OK, I don’t believe it’s discriminatory.
If they were limiting users to one shot purely because they have a disability then it would be discrimination. They are limiting RAP to one shot because there are too many going down those particular queue lines and RAP users are complaining that it’s taking too long to get on the ride when returning for their slot.
It doesn’t make much sense to limit to one shot on a non peak day, but if you mention the queues at Chessington a lot of the rides require use of stairs to get on/off. Vampire being one.
People who choose to use RAP on a non peak day who are able to walk and stand ok around the queue line just because they are entitled to it are part of the wider problem in my opinion.