jonmorris0844 - I think perhaps what is meant is that it would be simpler if wheelchairs were all standard gauge, fitted with heating and enormous umbrellas, and able to use a corridor connection... and yes, I am only joking.
In the mean time, I would expect a reasonable balance to be struck. Having the wheelchair user board immediately after others have alighted from the train is usually the best solution (and already in place). If there is no room for "able-bodied" people to board at this specific time - due to crush loading - I don't see why a wheelchair user should be given priority to board, as that is going to remove people from the train who have also paid to use the service. However, if there is a reasonable way to make sure at least one space is clear by getting people to move down (and even with 455s at Clapham Jn, you can fairly often negotiate a small amount of room inside, even if the train is very full), I see no reason why a wheelchair should not be put there.
It would be nonsensical to prevent people from standing in a wheelchair space just because one might turn up later in the journey, and with that must come a certain level of understanding that sometimes those spaces will not be fully available later.
In the case at Clapham Junction this morning, I do wonder if some of those people standing on at least one of those trains couldn't have moved around inside, as soon as anyone alighting had been able to do so. I know just how ridiculously crowded those trains get, but you can usually find at least one doorway with room for 3 or 4 people, so that may well have been able to be used. However, that said, predicting where space will be on a packed 455 is sometimes impossible to do. Newer stock may soon be fitted with sensors to deal with this, but it's not going to help on many inner-suburban routes in the SWT area any time soon.