Thud,
I've been party to many similar musings over the years, so for what it's worth, here's my take on it:
Your IPP payments from 38 to 55 are about 150£k. your gratuity if you max commute is about 70£k which is actually worth 115£k if it were untaxed. By my reckoning then, if you leave before your IPP you loose a pre tax amount of approx £185£k. This figure would obviously need to be factored slightly because that 115k is spread over 17 years and gets erroded by inflation each year.On the otherhand, if you also factor in the extra cash you gain each year by chucking 70K at you mortgage at age 38 you gain a bit, but by that point my brain starts hurting.
Ultimately the location of the trap all depends on how much you plan on earning in your last years at the top level of seniority in an airline. Top whack F/L is about 64k ish. By joining an airline say 5 years early, you spend an extra 5 years of your working life earning nearer 100k (if you luck in!) So by my rough guestimation, I'd say about 5 years but I have plenty of mates who left with far less time than that to do simply because they couldn't put up with it all any longer. And for what it's worth, they seem happy doing the BA / Virgin thing rather than counting the pennies thay could have had in the long run.
Sorry I couldn't be more prescise as I think you were after a less vauge answer...what can I say, I learned everything I know from the crew room!!!
PS sorry about the spelling, in a hirry!!