I doubt that GB pilots would want to get involved in combining with BA pilots. They (GB)presently work for a company that is profitable and is sensibly managed by its owners. The franchise lasts for another 6 years, and then GB are free to work independently, or sign up with another airline(easyjet?airfrance?whatever).
Why get more closely involved with a once proud airline that is creaking with bad luck(sep11), militancy and mismanagement. BA has debts of £5.9bn and two-thirds of its income is being used just to pay the interest bills. BA can't afford to keep their present route structure on that kind of financial base - it has to shrink and that means less pilots. GB pilots would insist on being equals on a merged list to avoid being at the bottom of a shrinking combined business. Pruners all know that BA BALPA would not agree, because they wouldn't understand the significance of GB bringing a profitable and independent operation to the party.
The final point, of course, is that the owners of the airline call the tune at GB - not the workforce.