Under The Public Health (Ships) Regulations, what restrictions apply on boarding or leaving a ship after arrival at a UK port?

Where the authorized officer so directs, or where the master is required to make a report under Regulation 13 (see Preparations for health clearance in Section H), no person other than the pilot, a customs officer, or an immigration officer, may, without the permission of the authorized officer, board or leave a ship until free pratique has been granted, and the master must take all reasonable steps to secure compliance with this requirement.