Article 9A of the ANO
(b) The CAA shall refuse to issue a permit to fly in respect of an aircraft if it appears to the CAA that the aircraft is eligible for and ought to fly under and in accordance with a certificate of airworthiness.
Various reasons why an aircraft may not be eligible for a C of A (non-compliance with the Type Certificate probably being the major one, as in the case of the Ex-Mil Gazelles)