What shall I do, just wait for them to chase me / send to debt collectors? Buying a house currently, so don't want anything that could cause issues there.
As I see it you are actually in a super strong position - I'll explain my reasoning;
There are several parking firms who 'manage parking' within the areas of statutory control at airports in this country. They are all well aware of the limitations of PoFA in those circumstances but you'd be surprised how many of those companies 'try it on' in terms of their wording on their NtKs. Some claim that PoFA is applicable and some claim that they still have, 'the right to assume that the Keeper was the driver' in order to try and pass liability onto a named individual.
What you notice with all these companies is a reluctance to pursue Keepers all the way to a physical hearing - and there is a very good reason for that;
In simple terms, they risk their dubious behaviour being found out. They risk awkward cross-examination. They risk their non-compliance with legislation being exposed. Etc etc.
In your particular case, it is clear that MET have made little effort to establish the true area of statutory control. If you look back at previous cases then you'll see how MET's evidence (regarding 'relevant land') changes from time to time.
SO HOW CAN THEY ISSUE A LAWFUL NTK IF THEY DON'T EVEN KNOW THE NATURE OF THE LAND WHICH THEY ARE TRYING TO MANAGE? (Sorry to shout but it's a vital point.)
So let's imagine that they progress this to a County Court hearing... What's going to happen when you hold them to strict proof that the area is not under statutory control? The just stop oil map found online by the work experience kid is not going to cut it.
At the moment they are on a nice little earner by hoodwinking people into thinking that PoFA applies - do you think they are going to risk all of that over one case?
Looking at FTLA (and other parking related internet sites) it is obvious that this is a high earning site for MET - imagine how that would change if the true nature of the land was correctly established?