The requirements imposed in respect of importation, exportation and coastal trading under this Act shall not apply—
(a) to any vessel or aircraft owned by or in the service of the Government when being used for the purpose of customs, police or coastguard; and
(b) where the Comptroller so directs, and for such periods and subject to such conditions and restrictions as the Comptroller may see fit to impose, to any vessel or aircraft owned by or in the service of the Government of any other country.