Welcome to FTLA.
A good 'Plan A' is to contact the manager of the Waitrose - issuing parking charges to loyal customers doing a big shop is hardly conducive to repeat custom. If doing this, do so in your capacity as the keeper, and do not tell them who was driving.
Plan B: For reasons known only to them, Britannia have made no attempt to comply with the requirements of Schedule 4 of the Protection of Freedoms Act, and accordingly, cannot recover the charge from the registered keeper. The keeper can therefore appeal the charge with 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 28 days, chase them.