21. An NLS ship other than a United Kingdom NLS ship must not be enabled to proceed to sea from a port in the United Kingdom unless—
(a)a Contracting State has issued, and where appropriate endorsed, a relevant NLS certificate in respect of that ship and that certificate (and, where appropriate, that endorsement) is still valid;
(b)the Government of a State which is not a Contracting State has issued, and where appropriate endorsed, a certificate in respect of that ship which is deemed by the Certifying Authority to have the same force as a certificate issued in accordance with the requirements of Annex II and that certificate (and, where appropriate, that endorsement) is still valid;
(c)a surveyor of ships—
(i)has carried out a survey of the ship as if regulation 9 applied to that ship; and
(ii)is satisfied that the ship can proceed to sea without presenting an unreasonable threat of harm to the marine environment; or
(d)a person having power to detain the ship—
(i)is satisfied that the ship can proceed to sea for the purpose of proceeding to the nearest appropriate repair yard available without presenting an unreasonable threat of harm to the marine environment; and
(ii)has permitted the ship to so proceed.