The GSS arrangement requires all BAWC flying to be returned in-house (probably with the co-pilots) if the operation ever requires more than four (?) B744s. My understanding is that was the best deal at the time. That protection still stands. Not ideal but there it is.
Perhaps if BA had been receptive to a similar arrangement for OS (something like if it ever requires more than the initial six aircraft, it reverts to mainline), we may not have found ourselves in the current predicament. Unfortunately, that would prevent BA from doing a QANTAS/Jetstar, which is probably why it never came up.