Acquisition by descent.
(1)A person born outside the United Kingdom [F17and the qualifying territories] after commencement shall be a British citizen if at the time of the birth his father or mother—
(a)is a British citizen otherwise than by descent; or
(b)is a British citizen and is serving outside the United Kingdom [F18and the qualifying territories] in service to which this paragraph applies, his or her recruitment for that service having taken place in the United Kingdom [F19or a qualifying territory]; or
....