If they have admitted their fault in letters to you file a small claims court claim. It is cheap to do so and you can do so online. The link is here....
https://www.moneyclaim.gov.uk/web/mcol/welcome
Jet2 are responsible for your losses, provided you acted reasonably to mitigate them. It will come down to a judgement as to whether you can establish that abondoning your property was a reasonable response to the situation. It's borderline... The property you abandoned was worth more than they were asking you to pay. Had you paid, you would have been entitled to a full refund and everyone would have been made whole. Your decision to abandon the property could be construed as an error of judgement. If you can establish that you didn't have the means to pay at the time, or had another good reason for your actions, you will win. Jet2 breached your contract.