Take Dragonair from Shanghai to Hong Kong, then transfer to London, and then transfer to Aer Lingus to Belfast. Why do you need to clear customs for your luggage in London?

You are only transiting for international connections in Hong Kong and do not leave the international hall, so there is no customs clearance problem. After arriving in London, you will transfer from the international hall to the domestic hall, which means that you entered the UK in London, so you must clear customs in London. The flight from London to Belfast is a domestic flight, and there is no customs clearance problem. . . . .