Sorry, not my area of expertise I'm afraid. My previous comments were just based on what a former colleague was told when he got his LoL settlement.
I don't think you'll be able to claim the PILON as tax free as it isn't a contractual redundancy payment, primarily because your termination is not due to your role being made redundant.
From what I have heard though, there are a number of airlines in discussion with HMRC over whether LoL premiums should be treated as benefits in kind...and have been in discussion for a number of years with no agreement reached with HMRC.
There is a chance you might be able to have some success but I think it will be a long process, potentially expensive (legal fees etc) and time-consuming. If you want to pursue it I would definitely seek advice from an employment lawyer though as they would hopefully have a good idea of your chance of success.
Best of luck though as success would help out a lot of people in similar situations at a time when money could be especally precious for them.