No one pays UKPC if they follow the advice. Please show the Notice to Keeper (NtK) or the Notice to Driver (NtD) if it was a windscreen PCN.
Never ever use a signed for service to send anything to these scammers. If it wasn't signed for, then all you have is proof that it was not delivered. Not much use.
When we've seen the PCN we can advise on the best appeal and subsequent POPLA follow up.
Have you tried asking the management (not the customer service staff) at the location to get them to get it cancelled?
Also, it appears that there is no bay to the right of the vehicle or it is blocked with some kind of construction. If so, you can see how ridiculous the allegation is.
Let's get a bit more detail first.