Actually it's an internal application and you're guaranteed to pass the application stage, from there you have to pass the various assessment stages like everyone else.
It's true new starters are held back 18 months, and newly promoted captains 12 months to stem the flow. But considering a two or three year type rating bond that seems fair enough. It's also known hold poolers are further blocked if crew levels at BACF are more critical than BA's need for pilots - a fine line at the moment.
Things will probably change as we are haemorrhaging Captains to Longhaul at the moment.