Right now there is nothing more you can do about this. You can safely ignore all debt recovery letters. Debt collectors are powerless to do anything except to try and persuade the low-hanging fruit on the gullible tree to pay up out of ignorance and fear.
Never, ever, EVER, enter into communication with a useless debt collector. They are not a party to the contract allegedly breached by the driver and you must never interact with these bottom-dwelling firms.
For now, we would need to see the Notice to Keeper (NtK) you received. Only redact your personal details, the PCN number and your VRM. Leave everything else visible, especially all dates and times.
Eventually you are going to receive a Letter of Claim (LoC). Come back when you receive that and we can advise on how to respond. If you follow the advice given here, you won't be paying a penny to UKPS.