It may be BA's policy that pax should have the first choice of on board duty free but that's not what happens in practice, at least not in my experience of many flights in CE to and from KBP. What happens in practice is that after the meal service on the outbound, the duty free trolley is wheeled to the front galley, the curtain is drawn, and the flight and cabin crew take their pick of the duty free. When they've finished the trolley is wheeled back through the cabin and pax get to choose from whatever is left.