Oh dear! FUBAR! All the Keeper had to do was put the operator to strict proof that any breach of the terms occurred and then refer them to the answer given in
Arkell v Pressdram (1971). Sadly, that appeal gave them their decision on a plate with a cherry on top.
What evidence do you think they have that the driver left the premises? Their Notice to Keeper (NtK) is not PoFA compliant so they cannot hold the Keeper liable if the driver is unknown.
Never mind, you are where you are and all that means is that you will now have to go through the lengthy process of ignoring all their useless debt recovery letters and wait for them to issue a county court claim, where this will easily be defeated when they discontinue just before they have to ay the trial fee.
The POPLA decision is not binding on the Keeper and you should not pay it. You a safely ignore all debt recovery letters as debt collectors are powerless to do anything except to try and intimidate the low-hanging fruit on the gullible tree into paying out of ignorance and fear.
Come back when you receive a Letter of Claim (LoC) and we will advise on how to respond. If you follow the advice given here, you will not be paying a penny to MET.