Marty112 decribes exactly the way Bassa run the operation in BA regarding crewing and disruptions. As an ex BA Capt., on a number of occasions I was unable to continue a legal operation after disruption to get my pax to their destination. This was soley due to BASSA imposed restrictions which BA Management were, and probably still are unwilling to overule.
I know it is often frustrating cessnapete but there are two signatures on all of those agreements and one of those was from whoever was running BA's side of things at the time! As a senior cabin crew member, all you do is read it and apply it; thats what BA teach us in training and thats what we do. Sometimes we would prefer to get home, finish the trip, etc but BA prefer that we work to the agreement. Away from base is easier because we can dip into Scheme and (that old favourite) Captains discretion! At base it does seem to make sense to ensure you have the right number of crew just in case you lose somebody later and risk stranding the a/c downroute.
Luckily, our operation is more robust since we moved to T5 and disruption less frequent. (The good old British weather is still out to get us and dear old LHR can't cope with a gust of wind or a flake of snow so I am sure I am in for an unpleasant week now I have said that!)