The relevant part of the ANO 2005 would appear to be:
(4) A person shall not be entitled to act as a member of the flight crew of an aircraft registered in the United Kingdom if he knows or suspects that his physical or mental condition renders him temporarily or permanently unfit to perform such functions or to act in such capacity.
(5) Every holder of a medical certificate issued under this article who—
(a) suffers any personal injury involving incapacity to undertake his functions as a member of the flight crew;
(b) suffers any illness involving incapacity to undertake those functions throughout a period of 21 days or more; or
(c) in the case of a woman, has reason to believe that she is pregnant;
shall inform the CAA in writing of such injury, illness or pregnancy, as soon as possible in the case of injury or pregnancy, and as soon as the period of 21 days has expired in the case of illness.
(6) The medical certificate shall be deemed to be suspended upon the occurrence of such injury or the expiry of such period of illness or the confirmation of the pregnancy; and—
(a) in the case of injury or illness the suspension shall cease upon the holder being medically examined under arrangements made by the CAA and pronounced fit to resume his functions as a member of the flight crew or upon the CAA exempting, subject to such conditions as it thinks fit, the holder from the requirement of a medical examination; and
(b) in the case of pregnancy, the suspension may be lifted by the CAA for such period and subject to such conditions as it thinks fit and shall cease upon the holder being medically examined under arrangements made by the CAA after the pregnancy has ended and pronounced fit to resume her functions as a member of the flight crew.