Use all the arguments you can at POPLA but the main one that will work is the fact that as long as the drivers identity is not revealed, they cannot transfer liability to you, the keeper.
All the other arguments in your appeal and attempted pleading with MET are a wasted effort once you come to the realisation that you are dealing with an ex—clamper firm of scammers. Their sole interest is in relieving you of your money.
Others on here may disagree with my views in this cowboy (Hansard) industry but many years of experience in assisting victims of these scammers has proven me right. MET, in particular, are one of the worst firms for trying on scams like this.
Fortunately for the OP, this occurred on airport land under statutory control and as long as they haven’t exposed themselves to be the driver in all the communications, they can simply lead the POPLA assessor to the fact that MET do not know who the driver was. They cannot rely on PoFA to transfer that liability.
The burden of proof that the keeper was also the driver is on MET. Unless they’ve been told by the keeper that they were also the driver, the POPLA assessor cannot show that the PCN has been correctly issued.
By all means, use all the other POPLA arguments but don’t pin your hopes on them. In the event that POPLA doesn’t accept your appeal, which can sometimes happen if the tea-boy is doing some of the assessments on the day, don’t worry about it. Let it go to court where a truly independent arbiter, a judge, will decide whether you owe a debt to MET.