We'll need to see images to be able to assist. The best way to upload images is to use a third party site, such as Imgur, or PostImg - we could do with seeing copies of the notice received, photos of the car in the space, as well as the wording of the appeal submitted, and UKPC's response.
Pursuing on the basis of a single wheel touching the line sounds like very sharp practice - a good first step would be to find out who owns or manages the retail park and speak to them to see if they will intervene. You should also begin drafting an appeal to POPLA (don't worry about the sentence about the Ombudsman service, POPLA provides the next stage of the appeal process) - POPLA aren't always great for issues like this and may well find in favour of UKPC, but a strong appeal will show UKPC that you're not going to be an easy target.
It's worth being aware that UKPC are known for playing brinksmanship with some of their cases - there have been a large number of instances in the past year or so of them pursuing charges as far as court - until defences have been submitted and hearing dates set, only to then discontinue when the defendant continues with their defence. This thread on the MSE forum highlights some examples:
https://forums.moneysavingexpert.com/discussion/6377263/dcb-legal-record-of-private-parking-court-claim-discontinuations
