Warning, it doesn't matter if you resign or retire.
You can also qualify for tickets without doing 15 years. Depends on when you joined, how many years you did and what age you will be when you resign. Information is readily available on the portal, go look it up.
Many permutations of entitlement, simply depends on the individual circumstances and what his/her needs will be as to whether it's worth hanging out for.
Will the firm tickets be available or reliable enough? I doubt it, treat them as sub load then you won't be disappointed.
Not the best retirement travel deal, but a long way short of the worst.