From someone who PVR'd many many years ago, the only Managed Path outside is that of supply and demand. Civil airlines, including BA, are all hostage to that simple equation and no amount of policy, published procedures, or understandings with "Manning" will overcome that fact of life. As others have said, if you've decided that the Service is no longer the future for you then you have made your choice. Have the courage of your conviction and PVR if that is the only way out.
BA is not the by all and end all anyway. In my day it was just another form of government work but not any more though, and it is fast becoming just another "independent airline" with a future as unknown and risky as any. I had a rewarding and fulfilling career with other carriers, until BA gobbled us up. Having seen them from inside and out, I preferred the latter scenario but that is a personal choice as will yours be. Whatever it is, good luck!