You're absolutely right — the "No Genuine Pre-Estimate of Loss" (GPEOL) argument has not been a standalone valid appeal point since the landmark Supreme Court judgment in ParkingEye Ltd v Beavis [2015] UKSC 67.
In Beavis, the Court held that a charge of £85 was not a penalty, even though it did not reflect a genuine pre-estimate of loss. The Court reasoned that a parking charge could be lawful if it served a legitimate interest and was not extravagant or unconscionable.
As a result:
• GPEOL died as a primary defence in 2015.
• It is no longer accepted by POPLA or courts unless you can show the charge is unconscionable or extortionate beyond the bounds of the Beavis case (which is difficult).
For that point, it would have been better to argue unfairness or lack of proportionality under the Consumer Rights Act 2015.
However, @skodabb has not been back to the forum since 30th March, so we may never know how they got on. Any MET Stansted PCN is easily dealt with as Keeper only because PoFA does not apply at the location.