My understanding of scope is not quite as you claim. I forget the exact wording but it essentially is an agreement that any future work flown in aircraft above a certain size, in BA colours will be flown by BA pilots. It says nothing about BA pilots going to franchises to fly.
The intent was rather that any such future work would as a result be kept "in house". The GSS situation was as a result of the apparent expansion of that operation. BALPA approached BA, pointed at the scop agreement and the compromise that was reached was the process whereby BA guys can bid to do that work on GSS T and Cs. This is not dissimilar to the situation that existed when BA pilots bid to work at what was then EOG flying the 737 on less pay than at lhr. Slowly that has resolved itself such that the work is on BA mainline pay with only the rostering yet to bring in to line with mainline. It's worth considering that had BALPA simply not allowed BA pilots to pursue this route in EOG the work would almost certainly have moved out of the BA fold permanently.
SCOPE is clearly far from perfect but unfortunately such agreements are not legally binding in UK law so inevitably compromises result....
At least that's how I understood it...