British Nationality and Status of Aliens Act 1943 s 3

s 3 British nationality of posthumous children

Any person born after the death of his father, whether before or after the commencement of this Act, shall, if his father died a British subject, be deemed to be and, in the case of a person born before the commencement of this Act, always to have been, a natural-born British subject in any case in which he would have been a natural-born British subject if his father had survived and remained a British subject until after the birth.