Do nothing until 11th July and then appeal only as the keeper. Make a note in your diary to do this. Do not say who was driving. You appeal as the keeper only. It cannot be stressed enough.
The reason for this is to tie them up in the appeals process at a time when they should also be posting a Notice to Keeper (NtK). Many PPCs in the BPA omit any NtK when replying to an appeal and the very fact a PPC forgets to send you a NtK by day 56, gives you a winning point at POPLA. Don't forget this tactic, and use it at POPLA stage.
Having appealed just before the 28th day after a windscreen PCN, make sure you take note of whether any postal NtK is ever received in the following weeks. A rejection letter with POPLA code is not a 'NtK'.
You must include 'no keeper liability' at POPLA stage if you do NOT receive a NtK at all in that time, and you will win as long as you make the missing NtK clear to POPLA.
Use the following as your appeal and either submit it as a PDF attachment to an email or use their web portal. Do not tick anything that suggests you were the driver. Select "keeper only" or "other":
Re PCN number: XXXXXXXX
I dispute your 'parking charge', as the keeper of the vehicle. I deny any liability or contractual agreement and I will be making a complaint about your predatory conduct to your client landowner.
There will be no admissions as to who was driving and no assumptions can be drawn. Since your PCN is a vague template, I require an explanation of the allegation and your evidence. You must include a close up actual photograph of the sign you contend was at the location on the material date as well as your images of the vehicle.
If the allegation concerns a PDT machine, the data supplied in response to this appeal must include the record of payments made - showing partial VRNs - and an explanation of the reason for the PCN, because your Notice does not explain it.
If the allegation involves an alleged overstay of minutes, your evidence must include the actual grace period agreed by the landowner.
In the meantime, continue with the complaint to the NHS Trust CEO and PALS.
If you still have the dash-cam footage make sure you keep a copy of it. If you can, it would be worthwhile showing us to see how bad the signage is. UKPC are renown for poor signage that breaches the BPA CoP.