I would guess that LHR passengers collect baggage in the international arrivals as the flights carry a lot of connecting international traffic from LHR, whose bags are cleared on arrival at the destination airport and not at by Customs at LHR. Terminal 1 at Manchester which is used by by BA flights from LHR does have a customs point and I have been checked by Customs there when I have arrived at MAN after connecting from an international flight at LHR.