There is no doubt that after the recent press exposé relating to the Home Secretary putting pressure on immigration to speed things up they are now doing very thorough checks. Clearly at certain times of the day when the arrivals schedule is at its peak this impacts on how quickly customers can pass through this area.
Baa have no input into this, infact by law the area itself is handed to immigration and they run it, with no input accepted from other groups (unless they want to hear it, and usually they don't).