But how are BA to know if a bag is connecting on two separate bookings? It doesn't register on the system because when you check your bags in at say BUD, the final destination for that booking is LHR. BA aren't magicians and can't predict that a bag tagged for LHR under one booking needs to get to ABZ on another separate booking. Either book your connecting flights together (which is also cheaper and allows compensation for missed connections) or deal with your own stupidity.