Bid Line
From memory.....BA keep and run the bidline type of rostering because it's cheaper that a normal roster. For the sake of paying a little overtime you never run out of pilots. There's always someone on a day off who is willing to work. You may have then to offer his trip to another (if he was due to work tomorrow) but that will work too. If there really is no volunteer then you 'force draft' from the junior upwards and that is a very nice little earner. Either way BA get a fully trained/qualified pilot 'off the street' with no extra training/holiday/pension costs. In that way you can run a tighter ship with less crews per fleet than other airlines.
Standby in the winter was mentioned. With a bidline you even bid for your month of reserve. There's no standbys on a work line. If you want to be busy bid for reserve over winter flu time. If you want time off to paint your house bid for reserve in May when there's hardly any sickness.
BA publish all the work (in work line rosters) and the pilots sort it out amongst themselves. That's a simplification but BA must be getting a lot from it or it wouldn't have been around since the early '70s. Personally I can't see it being ditched-tweaked, perhaps-anytime soon.