Sick, sorry to tell you this but I was in Cityflyer when BA was taking us over and merging us into mainline. TUPE doesn't give you the right to merged seniority (a bunch of CFE pilots set up a legal slush fund and took advice). Also, the BACC were adamant that seniority would not be (and subsequently wasn't) merged. The best outcome you could hope for is a "Grandfather Right" form of protection for existing commands.
F33