Even if you can't evidence having sold the vehicle before the date of the alleged contravention, you can easily defend as you were not the driver and there is no Keeper liability at the location because it is land under statutory control, Stansted Airport Byelaws.
MET will argue that it is private land and no one is disputing that. However, even if it is private land, it sits within the boundary of Stansted Airport and so is not "relevant land" for the purposes of PoFA. Only the driver can be liable.
FOr now, you need to respond to the Claim by submitting your Acknowledgement of Service (AoS). With a claim issue date of 22nd November, you have until Wednesday 11th December to do so. There is no advantage to delaying the AoS. By submitting the AoS before that deadline, you then have until 4pm on Friday 27th December to submit a defence.
TO submit the AoS, follow the instructions in this PDF document:
https://www.dropbox.com/s/xvqu3bask5m0zir/money-claim-online-How-to-Acknowledge.pdf?dl=0When you have done that, let us know and we will advise further on what to put in for the defence.
Here is the Stansted boundary map and the location of the alleged contravention:
