Whatever you submit for the initial appeal will be rejected, but there are other things you can throw in at POPLA that may have a better chance (although POPLA may not agree, as is their way

) - I can fish out an initial appeal for you to use later, unless b789 beats me to it.
A couple of initial observations that we can throw in later down the line at POPLA:
Entrance SignageThe entrance appears to be
here. The positioning of that sign is such that it could easily be missed - prominent entrance signage is important in all 'car parks', but even more important in cases like this, where there is no obvious indicator that you are leaving the publicly maintained highway and entering private land subject to parking controls.
Forbidding SignageFor a contract to exist there must be a number of elements including; an offer, consideration (both sides must exchange something of value), and acceptance. The signage you have showed us simply prohibits parking or waiting - no consideration is offered to park on certain terms. Without this, no contract can exist and you can argue this is simply an unenforceable penalty.
No Parking on Yellow LinesFor what purpose was the driver stopped on the yellow lines, and for how long? Loading/unloading of passengers is generally allowed on double yellow lines, it is waiting that is not allowed. Setting down or dropping off passengers is different to parking, so depending what happened an argument can be made that the vehicle was not parked.
No Stopping or Waiting The signage indicates a prohibition on stopping or waiting - how on earth is a motorist supposed to read the terms and conditions of the alleged contract and decide whether he accepts them if he is not allowed to stop? Someone cannot be bound by a contract if he has not had the opportunity to acquaint himself with the terms.