You have to appeal to Britannia first. You will only get an opportunity to appeal to POPLA if Britannia reject your appeal (which they will).
In your appeal to Britannia, irrespective of whether you paid or not, you should not be the liable person to pay. The liable person is the driver. Britannia have no idea who the driver is. You, as the keeper, are under no legal obligation to reveal the identity of the driver and Britannia are not allowed to simply presume or infer that because you are the keeper, you must also be the driver.
If the NtK had contained the all the required wording of PoFA and had been delivered within the required timescale, Britannia, because they don't know the identity of the driver, pass that liability to you, the keeper. However, they have failed to fully comply with the strict requirements of PoFA and so, cannot pass that liability on to you, the keeper.
Because they cannot rely on the provisions of PoFA to hold you, the keeper, liable, the burden of proof falls on Britannia to prove you were also the driver. The only way they can get that proof is if you tell them you were the driver. Remember, you are under no legal obligation to identify the driver, whether it was you or not.
So, appeal to Britannia with the following:
This is an appeal by the registered keeper - No driver details will be given. Please do NOT try the usual trick of asking for driver details in order to get around the fact your NtK does not fully comply with the strict requirements of PoFA. As there is no keeper liability, therefore, liability cannot flow from the driver to the keeper and so, is an automatic win at POPLA. Please cancel the notice or issue a POPLA code at which point you will auto withdraw.
No need to expend a lot of wasted effort at this stage as they will reject the appeal, no matter what. No need to inform them about which specific element of PoFA they have failed. They will just say that their NtK was PoFA compliant. It is at POPLA stage that your best chance of having this cancelled.