Oh dear, I'm assuming that you blabbed the drivers identity in your appeal. As the Registered Keeper, you are under no legal obligation to identify the driver, who is the only liable party. UKPC had no idea who that was unless you told them, inadvertently or otherwise.
Because their Notice to Keeper (NtK) is not fully compliant with ALL the requirements of PoFA 2012, they cannot hold the Keeper liable. The NtK does not refer to a "period of parking" as required by PoFA 9(2)(a). This could have been quashed at POPLA if the driver has not been identified.
However, do not despair. If you follow the advice, you won't be paying a penny to UKPC. It just means that this will now be a protracted affair and will only conclude in about 9-12 months after they have issued a county court claim and with our defence, will either be struck out or discontinued.
If the driver has not been identified, then this is easily winnable at POPLA. So, has the driver been identified to UKPC? For example, "I did this or that" instead of "the driver did this or that"?