If all of you received PCNs because none of you knew that it was controlled parking, then it is fairly obvious that the signs were not prominent nor capable of bringing the charge to the attention of the drivers. Looking on GSV, the newest photos of the entrance area are from 2022, possibly before PE were contracted. There were no signs at that time at the entrance to the facility.
Do you know anyone in the area that could get some up to date photos of the entrance area and the car park itself to see how prominent or not, the signs are?
Have you actioned Plan A yet? If you were patrons of the facility and you've been invoiced by an unregulated private parking company for the privilege, you may want to complain to David Lloyd and ask why it was not brought to your attention that it was a paid for car park.
When the time comes for Plan B, the appeal to PE, do not reveal who the driver was. Any appeal is done only as the keeper. The NtK does not fully comply with the strict requirements of PoFA 9(2)(e)(i) and therefore they cannot transfer liability for the charge from the driver to the keeper. PE will disagree and reject the appeal. However, you can use that for your Plan C POPLA appeal.
POPLA may or may not accept that point if it is not argued well and the assessor is not led by the nose to the strict requirements of PoFA. There are other arguments that will include signage and landowner authority etc.
For now, concentrate on Plan A and prepare your Plan B appeal and show us that before you send anything off to them You have until Tuesday 18th June to submit for that, so no rush.