You could have 100 PCNs issued as Notices to Keeper (NtK), you still cannot not be liable for any of them unless you also admit, inadvertently or otherwise, that you were also the driver. As there is no legal obligation to do so, why would you?
If you appeal only as the Keeper and it is rejected, all you do is ignore everything else that arrives in the post until they give up. You certainly don't respond any Debt Recovery Agent (DRA) letters. The DRAs are not a party to the contract allegedly breached by the unknown driver and are powerless to do anything except to try and scare the low-hanging fruit on the gullible tree into coughing up the dosh out of ignorance and fear.
Unfortunately, thanks to the ex-"dear leader", once the upcoming Private Parking (Code of Practice) Act 2019 comes into law, Keeper liability will also come into force in Scotland. So much for wanting "independence" from England.