Our usual recommendation for 'Plan A' is to ask the landholder, Waitrose in this case, to intervene and cancel. However, if he's struggling to lay his hands on a receipt to support an approach to them, the good news is that Britannia have failed to comply with the requirements of Schedule 4 of the Protection of Freedoms Act, and are therefore unable to hold your father liable as the keeper. 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 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"). You have chosen not to issue a Notice to Keeper in accordance with The Act, and it is now too late for you to do so.
There is no obligation for me to name the driver and I will not be doing so. 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, be careful there are no drop down/tick boxes that cause you to identify who was driving, and keep a close eye on your spam folder for their response. If they do not respond within 35 days, chase them. The appeal must come from whoever the notice is addressed to, so if your father is the registered keeper and you're submitting on his behalf, enter his details as the keeper.