My initial view is that they are right.
I understand what you are saying about it being parked on the road
But if you go on holiday and its at an airport, you are generally only away for a few months.
In any event you would have a contract with the airport car park to look after your car for a period of time when it was parked.
Here its been 12 months.
If it had been your own garage, its a different issue.
I think you need to read the terms carefully. See what it says about the definition of 'keeping possession'.
But my view is that it is only land not belonging to you and has been so for a year. I can't see how you can argue its in your possession because of the above.
I am sorry if this is not the answer you want, nor the one I want to give you, but I have a duty to be honest
Can I clarify anything for you about this today, please?
If you need anything further I am available for a follow up at no extra cost.