Welcome - good that you chose to seek advice here before appealing.
The set-up at Heathrow is such that there is no mechanism for APCOA to recover any charge that may be owed by the driver (who APCOA do not know), from you, the keeper. Schedule 4 of the Protection of Freedoms Act (there's a link to it in my signature under this post if you want to read up on it) provides a mechanism for parking companies to recover charges from the registered keeper of a vehicle, if certain conditions are met. It states that they can only do this if the alleged parking event took place on "relevant land", which is defined in paragraph 3 as follows:
3(1)In this Schedule “relevant land” means any land (including land above or below ground level) other than—
(a)a highway maintainable at the public expense (within the meaning of section 329(1) of the Highways Act 1980);
(b)a parking place which is provided or controlled by a traffic authority;
(c)any land (not falling within paragraph (a) or (b)) on which the parking of a vehicle is subject to statutory control.
As Heathrow Airport is subject to byelaws, it is subject to "statutory control", and as such, not relevant land. You could therefore appeal along the lines of the below:
Dear Sirs,
I have received your Parking Charge Notice (Ref: ______) for vehicle registration mark _______, in which you allege that the driver has incurred a parking charge. I am appealing as the registered keeper of the vehicle. There is no obligation for me to name the driver and I will not be doing so. I note from your correspondence that you are not seeking to hold me liable as the registered keeper, under Schedule 4 of the Protection of Freedoms Act 2012 ("The Act").
Even if you were seeking to do so, as the Heathrow Airport Drop Off Zones are not "relevant land" as defined by The Act you are unable to recover the charge from me, the keeper.
I am therefore unable to help you further with this matter, and look forward to your confirmation that the charge has been cancelled. If you choose to decline this appeal, you must issue a POPLA code.
Yours,
If appealing online, make sure there are no drop down boxes etc. that force/trick you into revealing who was driving - you are appealing as the keeper, and saying nothing about who was driving.