OK. So, at any point in your appeal did you refer to yourself as the driver? Using terminology that says "I did this or that" instead of referring to the the driver in the third person such as "The driver did this or that" is how you inadvertently blab the identity of the driver being the person making the appeal.
So, did you throw away the easiest of all PCNs to dispose of by identifying the unknown (to APCOA) drivers identity as also been the same as the known Keeper?
Either way, luckily for you, APCOA are benign and will not litigate. However, if you have thrown away the easiest of PCNs to dispose of by blabbing the drivers identity, you must now go through the inevitable storm of reminders and Debt Recovery Agent (DRA) letters. Do not be low-hanging fruit on the gullible tree and freak out at the scary letters. DRAs are not a party to the contract allegedly breached by the driver and have no power to actually do anything. Never, ever, ever communicate with a powerless DRA. Ignore them.
If you follow the advice, APCOA will eventually give up and move on to even more low-hanging gullible fruit who are likely to pay out of ignorance and fear.
Next time you forget to pay APCOA at Luton or any of the other airports they infest, remember that as long as the known Keeper doesn't blab the unknown drivers identity, there is absolutely nothing they can do about it and a simple appeal reminding of that fact and why, will result in a PCN cancellation.