As jfollows rightly notes, too late for PoFA compliance. The keeper could send a standard PoFA based appeal such as the one 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 them to identify who was driving, and keep a close eye on the spam folder for their response. If they do not respond within 28 days, chase them.
Britannia are sometimes an awkward bunch, so if they do reject the appeal, I'd be confident that a POPLA appeal will see it off.