Other airlines manage this fairly consistently (I’m thinking CX, QF, LH for example) so it can undoubtedly be done. LH for example evaluates station managers on this performance criterion.
So why can’t BA get this right?
Because BA chooses not to spend money on this. It really is that simple. BA doesn’t value its premium passengers’ time highly. The feeling is mutual (hence Ba’s lack of pricing power in F on routes with serious competitors.)