You won't be paying a penny to UKCPM if you follow the advice. Are you sure you did not receive the original NtK? It would be worthwhile seeing both sides of that as it is the key document to be able to see whether all the legal requirements to hold you liable as the keeper.
A few more questions though first. Was the driver or passenger in possession of a blue badge, irrespective of whether it was shown or not? At what time of day was this? Daylight or darkness?
The first thing that stand out are the signage is terrible. It is almost impossible to read that wall of text and there is no way that the charge is adequately brought to the attention of the driver. Secondly, loading/unloading or picking up/dropping off a passenger is not parking and that is defined in the persuasive appeal court judgment of
Jopson v Homeguard B9GF09AE [2016].
Unfortunately, you are dealing with a particularly vexatious company of ex-clamper thugs and it is too late to do any appeals. You will have to weather a storm of debt collector letter which you can safely ignore. They are powerless to do anything, no matter how nasty they threaten you. Just file them or use them as kindling.
You will have to wait until/if/when they decide to make a claim against you. Whilst we haven't seen the original NtK, it is advised that any communication we suggest is made as the keeper of the vehicle. UKPCM have no idea of the drivers identity unless you, the keeper tells them. The keeper and the driver are two separate entities in civil law and no inference or assumptions can be made that the keeper was also the driver.
Was the person that was dropped off at the location an owner or tenant? If so, what does their lease/AST say about parking? What it doesn't say is equally important.
Anyway, a few questions for you to answer in there before we move on.