Whilst it could have easily been appealed at the time by the Keeper with a simple statement advising APCOA that the Keeper is under no legal obligation to name the driver and declines to do so, and that would have been the end of it.
However, it is too late now to appeal. APCOA are benign and will not escalate this to a court claim. However, you will continue to receive useless debt recovery letters which you can safely ignore. Debt collectors are powerless to do anything except to try and persuade the low-hanging fruit on the gullible tree to pay up out of ignorance and fear.
Never, ever, ever, communicate with a powerless debt collector. Shred their letters and use them as hamster bedding, if you must.
Get on with your life and ignore everything about this. It will eventually stop.