This thread raises interesting issues of propriety because the driver made reps so promptly and the authority responded similarly.
OP, at no point in these proceedings could you, as driver,ever be liable.
Whether you are the registered keeper is legally irrelevant prior to an NTO being issued.
A PCN of this type is NOT a demand for payment, it's information contained in a notice. This info notifies the driver that the OWNER is liable but that if the penalty is paid - by anyone, not just you - then the matter would be closed, otherwise the authority would demand payment of the regular penalty from the OWNER. Very similar to, but the upside down version of, private parking cases where a Notice to Keeper is sent to the keeper to tell them that the driver is liable!
Anyway, the website is totally misleading when it refers to penalties owed by the keeper who cannot be assumed at this stage i.e. prior to a NTO, to know anything about matters. It's total c**p.
It is totally at variance with propriety for the council to pass info to the person who is NOT liable threatening them with higher penalties than permitted which would in any case be owed by the OWNER. Until a NTO is served then the OWNER could not access the site for want of the magic formula: the PCN number.
The authority really need to get someone in to take a look at their procedures and the assumptions which they make regarding the status of who might be viewing.
How could the actions or inaction of the recipient of a PCN give rise to any action or threat of action by the authority against the liable party who prior to a NTO being issued could have no knowledge and has even less liability?
It's procedural nonsense IMO.