United Kingdom of Great Britain and Northern Ireland

(1) A person born in the United Kingdom after commencement or in a qualifying territory on or after the appointed day, shall be a British citizen if at the time of the birth his father or mother is—
(a) a British citizen; or
(b) settled in the United Kingdom or that territory.
(1A) A person born in the United Kingdom or a qualifying territory on or after the relevant day shall be a British citizen if at the time of the birth his father or mother is a member of the armed forces.
(2) A new-born infant who, after commencement, is found abandoned in the United Kingdom, or on or after the appointed day is found abandoned in a qualifying territory, shall, unless the contrary is shown, be deemed for the purposes of subsection (1)—
(a) to have been born in the United Kingdom after commencement or in that territory on or after the appointed day; and
(b) to have been born to a parent who at the time of the birth was a British citizen or settled in the United Kingdom or that territory.
(3) A person born in the United Kingdom after commencement who is not a British citizen by virtue of subsection (1), (1A) or (2) shall be entitled to be registered as a British citizen if, while he is a minor—
(a) his father or mother becomes a British citizen or becomes settled in the United Kingdom; and
(b) an application is made for his registration as a British citizen.
(3A) A person born in the United Kingdom on or after the relevant day who is not a British citizen by virtue of subsection (1), (1A) or (2) shall be entitled to be registered as a British citizen if, while he is a minor—
(a)his father or mother becomes a member of the armed forces; and
(b)an application is made for his registration as a British citizen.
… (British Nationality Act 1981, Sec. 1)

(1) A person born outside the United Kingdom and 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 and the qualifying territories in service to which this paragraph applies, his or her recruitment for that service having taken place in the United Kingdom or a qualifying territory; or
(c) is a British citizen and is serving outside the United Kingdom and the qualifying territories in service under an EU institution, his or her recruitment for that service having taken place in a country which at the time of the recruitment was a member of the European Union.
… (British Nationality Act 1981, Sec. 2)²

(1) If while a person is a minor an application is made for his registration as a British citizen, the Secretary of State may, if he thinks fit, cause him to be registered as such a citizen.
(2) A person born outside the United Kingdom and the qualifying territories shall be entitled, on an application for his registration as a British citizen made while he is a minor, to be registered as such a citizen if the requirements specified in subsection (3) or, in the case of a person born stateless, the requirements specified in paragraphs (a) and (b) of that subsection, are fulfilled in the case of either that person’s father or his mother (“the parent in question”).
(3) The requirements referred to in subsection (2) are—
(a) that the parent in question was a British citizen by descent at the time of the birth; and
(b)that the father or mother of the parent in question—
(i) was a British citizen otherwise than by descent at the time of the birth of the parent in question; or
(ii) became a British citizen otherwise than by descent at commencement, or would have become such a citizen otherwise than by descent at commencement but for his or her death; and
(c) that, as regards some period of three years ending with a date not later than the date of the birth—
(i) the parent in question was in the United Kingdom or a qualifying territory at the beginning of that period; and
(ii) the number of days on which the parent in question was absent from the United Kingdom and the qualifying territories in that period does not exceed 270.
…
(5) A person born outside the United Kingdom and the qualifying territories shall be entitled, on an application for his registration as a British citizen made while he is a minor, to be registered as such a citizen if the following requirements are satisfied, namely—
(a) that at the time of that person’s birth his father or mother was a British citizen by descent; and
(b) subject to subsection (6), that that person and his father and mother were in the United Kingdom or a qualifying territory at the beginning of the period of three years ending with the date of the application and that, in the case of each of them, the number of days on which the person in question was absent from the United Kingdom and the qualifying territories in that period does not exceed 270; and
(c) subject to subsection (6), that the consent of his father and mother to the registration has been signified in the prescribed manner.
(6) In the case of an application under subsection (5) for the registration of a person as a British citizen—
(a) if his father or mother died, or their marriage or civil partnership was terminated, on or before the date of the application, or his father and mother were legally separated on that date, the references to his father and mother in paragraph (b) of that subsection shall be read either as references to his father or as references to his mother; and
(b) if his father or mother died on or before that date, the reference to his father and mother in paragraph (c) of that subsection shall be read as a reference to either of them.
(British Nationality Act 1981, Sec. 3)³