You were not issued a PCN by DCB Legal. You were issued the PCN as a postal Notice to Keeper (NtK) by UKPC. Show it to us, suitably redacted of personal information, PCN number and VRM. However leave all dates and times showing.
UKPC NtKs tend to be PoFA compliant so having blabbed the identity of the driver does not really affect much and the Keeper can still be liable. However, the cardinal sin you breached was making any form of contact with DCBL (not DCB Legal) who are powerless debt collectors and should be ignored.
DCB Legal are a firm of bulk litigators who are acting on behalf of UKPC to sue you in the county court for the alleged debt. Very easily defended and will, with more than 99% certainty, eventually be discontinued early next year.
What you have received is an N1SDT claim form from the CNBC. Again, show us that form, only redacting your personal details, the claim number, your VRM and the MCOL password. Make sure ALL dates remain visible. We don’t need to see any of the other forms that came with the claim and you can dispose of those as you will not be needing them.
Once we have seen the claim form, we can advise on deadlines for submitting the Acknowledgement of Service (AoS) and the defence. Do not do anything until you have shown us.
A couple of questions and points… was the McDs open at the time? Did you make a purchase? How long was the vehicle on the premises? It matters not whether the engine remained on, the lights remained on or whether anyone remained in the vehicle. The Keeper is being sued because the driver breached a term of contract on the signage at the location.