Any person appointed to be solicitor or assistant solicitor of Her Majesty's Customs, or any clerk duly appointed to act on his behalf or under his or their directions, shall and may in any case relating to the Customs, or under the direction of the Commissioners of Her Majesty's Treasury or Customs, act as counsel, solicitor, attorney-at-law, advocate, or writer to the signet in the prosecution, conduct, or defence of any such case in any court, jurisdiction, or place in which such case may be instituted, and any such solicitor, assistant solicitor, or clerk, and any officer of Customs, under the order and directions of the Commissioners of Customs, may prosecute, defend, or conduct any proceeding before any justice in any matter relating to the Customs to be heard or determined by him.