Looking in from the outside (I'm neither BA or BMI), the thing that seems obvious is that from what I hear, on day one at BA you are indoctrinated with the mantra "seniority is everything", which is proven to be true every day of your entire career at BA.
It seems somewhat naive of the BMI pilots tothink that the BA pilots would allow any threat to the very fundamental of their working life.
One can only imagine the fallout if a BMI pilot took the matter to court and, in doing so, destroyed the seniority list at BA...