Oh dear... attempting to have a conversation with APCOA. Not what we would advise.
Anyway, you have not received any "fines" so please stop calling them that. For every occurrence of the word "fine" that you can find on your Notice to Keeper (NtK) I will give you £100.
The advice we give to anyone in receipt of an NtK from APCOA for anything to do with Heathrow is as follows, assuming you have not blabbed that you were the driver:
I am the registered keeper. APCOA cannot hold a registered keeper liable for any alleged contravention on land that is under statutory control. As a matter of fact and law, APCOA will be well aware that they cannot use the PoFA provisions because Heathrow Airport is not 'relevant land'.
If Heathrow Airport wanted to hold owners or keepers liable under Airport Bylaws, that would be within the landowner's gift and another matter entirely. However, not only is that not pleaded, it is also not legally possible because APCOA is not the Airport owner and your 'parking charge' is not and never attempts to be a penalty. It is created for APCOA's own profit (as opposed to a bylaws penalty that goes to the public purse) and APCOA has relied on contract law allegations of breach against the driver only.
The registered keeper cannot be presumed or inferred to have been the driver, nor pursued under some twisted interpretation of the law of agency. Your NtK can only hold the driver liable. APCOA have no hope at POPLA, so you are urged to save us both a complete waste of time and cancel the PCN.
Your saving grace is that if you have revealed the drivers identity, APCOA are not litigious and apart from useless debt collector letters which should always be ignored, nothing will happen.