As Heathrow Airport is subject to byelaws, it is subject to "statutory control", and as such, not relevant land for the purposes of Schedule 4 of the Protection of Freedoms Act (there's a link in my signature), an act that allows parking companies to hold keepers liable for parking charges.
As a result, only the driver (who they do not know) could be liable. 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 T2 Drop Off Zone is 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. If you are not the registered keeper of the vehicle, with the V5C in your possession, tell us, as the appeal will be different.
So far this appeal has a 100% success record.