Edit: clearly crossed with b789's postPersonally, I'd ignore, let the 14 days elapse, and wait for them to either cancel the charge or issue you a POPLA code.
I would also be minded to immediately lodge a complaint with the British Parking Association, attaching a copy of the 2 letters sent on the same day, pointing out that Britannia are saying that the notice is not issued using the provisions of PoFA, then in another letter claiming that you are being notified of keeper liability under Schedule 4 of PoFA. We can help you draft something (although I don't have much time immediately).
If you must reply to Britannia, I'd keep it short and sweet:
Dear Sirs,
We are surprised to receive two letters, issued on the same day, which directly contradict one another. The first is a near-verbatim duplicate of your previous correspondence, confirming that the parking charge is not PoFA compliant, whilst the second letter bizarrely claims that Britannia have the right to recover the charge from us as the registered keeper, under PoFA. Both statements cannot be true.
As you have clearly failed to properly read or address our previous comments, I refer you to our previous letter of [DATE]. Our position remains unchanged: cancel the charge or issue a POPLA code.
Yours...