Where a ship has been within the limits of any port F2in the United Kingdom or the Isle of Man, F3a railway vehicle has been within the limits of a railway customs area or an aircraft has been in the United Kingdom F2or the Isle of Man, with a cargo on board and a substantial part of that cargo is afterwards found F2in the United Kingdom to be missing, then, if the F4vehicle operator fails to account therefor to the satisfaction of the Commissioners, F5the ship, railway vehicle or aircraft shall be liable to forfeiture.