Pity you did not come here first before doing anything. As this was a windscreen Notice to Driver (NtD), you have done all the dirty work for ECP and given them what they could never have achieved, on a silver plate with cherries on top!
Their NtD is not PoFA compliant which means that they could never transfer liability to the Keeper if the driver is not identified. They had absolutely no idea who the driver was until you blabbed it to them.
All you had to do is respond to the NtD on day 27 after the event with an appeal by the Keeper stating that as the Keeper, they cannot hold them liable as their NtD did not comply with PoFA 7(2)(a). They would have either responded and rejected that appeal without ever issuing a Notice to Keeper (NtK), another requirement under PoFA 8(4) to be able to hold the Keeper liable if the driver is not identified.
Either way, the Keeper could have appeal to POPLA and it would have been cancelled. However, that was not done and now the driver is identified so the wining PoFA argument is flushed down the toilet.
Any POPLA appeal now is futile. However, all is not lost. As this is an ECP PCN, it will escalate all the way to a county court claim issued by their preferred bulk litigator, DCB Legal. I can assure you with greater then 99.9% certainty that any claim issued by DCB Legal, as long as it is defended with out template defence, will eventually either be struck out or discontinued.
Just do a search of the forum for any other ECP PCN, to see the process. Unfortunately for you, what would have been a simple 5-8 week process ending with a cancellation will now be a 9-12+ month process, albeit still ending without you paying penny to ECP.
I don't advise wasting you time with a futile POPLA appeal. You can safely ignore all the useless debt recovery letters that will come your way. 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) from DCB Legal and we will take it from there.