As soon as you have the rejection from the MET appeals department you can go to POPLA. In the rejection document they will have put your unique 10-digit verification code for your MET case to use with POPLA (Near the end in section 2 marked in black box below ).

https://appealform.popla.co.uk/#/StartAppeal/Index
1: Set the “Grounds for Appeal” reason to “Other”
You may need to alter this next appeal depending on the response they give you. If they stated its private land and not subject to any bylaws, you can confidently just copy paste the below argument (after the annotation bubble). You again will need to provide that image of the bondary from Step 1 to the POPLA appeal
The reason for your appeal
Please use this to provide an overall summary of the reasons for appealing against the parking charge notice.
I am the registered keeper of the car. The MET wants to hold me, the keeper, liable for breaches of conditions on private land. I appealed because the Southgate parking lot falls within the boundary of Stansted Airport. The parking was established after the airport boundary was defined. I requested the MET to prove or get confirmation from Manchester Airports Group that this parking was not subject to any statutory scheme.
The reply I received was that it’s Private land, which, in my opinion, is not sufficient evidence, and I hope the POPLA agent will agree with this and ask the MET to provide official documentation or a statement from “Manchester Airports Group”(MAG) supporting the MET’s claim that the Southgate Park does not fall under MAG’s statutory regulations.
Please see below the argument I put forward.
Southgate Park is within the boundary of Stansted Airport. Please see the boundary picture taken from public records attached. The red mark indicates the location of Southgate Park, Stansted CM24 1PY. This clearly shows the park within the boundary of the airport designated site land. (See attached image) Please provide evidence that this location is designated as “relevant land” and not governed by a statutory scheme, which is applied and governed by the airport owner/operator, in this case, Manchester Airports Group. Protection of Freedoms Act 2012 (Ref: https://www.legislation.gov.uk/ukpga/2012/9/notes/division/5/1/3/3) Keeper liability under Schedule 4 applies only to “relevant land.” Paragraphs 2–3 of Schedule 4 make clear that the scheme is restricted to “relevant land.” “Relevant land” expressly excludes: (a) highways maintainable at public expense, (b) parking places provided or controlled by a traffic authority, and (c) any land where parking is governed by a statutory scheme, including under Part 6 of the Traffic Management Act 2004. Land under statutory control is therefore not “relevant land.” Paragraph 220 explicitly states: “Other land where parking is governed by a statutory scheme … is also excluded from the scheme as set out in this Schedule.” This means if the land is subject to statutory regulation (for example, council-controlled car parks, or land where parking enforcement is governed by by-laws or legislation), it is excluded from Schedule 4 altogether. If land is not “relevant land,” the statutory mechanism for keeper liability does not apply. Paragraph 221 provides that a creditor has the right to recover unpaid parking charges from the keeper only if the conditions in paragraphs 5, 6, 11 and 12 are satisfied. However, the starting point is that this recovery right applies only to “relevant land.” If the land falls outside the definition (i.e., it is under statutory control), then Schedule 4 cannot be invoked to transfer liability from the driver to the keeper. Legal consequence: On non-relevant land, the driver alone may be liable (either by contract or by committing trespass). The keeper cannot be held liable under the Protection of Freedoms Act 2012, because Parliament deliberately excluded land under statutory parking schemes.
Expect to be harrased by PCN scammers. Once you have completed this.
My theroy is that soon as the MET knows you intend to challenge it with a possible winning argument they will sell your data onto scammer groups to extract some sort of value off the ticket. (Expect this near the end of the POPLA challenge to MET)

