What did PALS say when you approached them about this? Have you emailed the CEO of the NHS Trust to complain about the Pan and the fact that whoever it was you did speak to fobbed you off?
Have they followed all the guidelines from this document:
NHS car parking guidance 2022 for NHS trusts and NHS foundation trusts (https://www.gov.uk/government/publications/nhs-patient-visitor-and-staff-car-parking-principles/nhs-patient-visitor-and-staff-car-parking-principles#)
In particular:
Contracted-out car parking
NHS organisations are responsible for the actions of private contractors who run car parks on their behalf.
NHS organisations should act against rogue contractors in line with the relevant codes of practice where applicable.
Contracts should not be let on any basis that incentivises additional charges, for example ‘income from parking charge notices only’.
UKPC are a BPA member and so the BPA Code of Practice applies. There is a link to their CoP in @DWMB2s signature. Have a look at bits about signage and whether what you saw at the location complied with the requirements as set out? Was it prominent? Was whatever was prominent fully legible (UKPC signs never, ever, comply with the CoP. What is obvious that you were parked in a restricted area? How was it delimited?
No one should pay a UKPC charge. If it ever got as far as a claim, they would discontinue before it ever got to a hearing.