United Kingdom - Passport & Nationality - British Citizenship by Birth (British Parent)
A claim for British Citizenship by birth can be made where your child:
a) was born in the UK; AND
b) is still under 18
where a parent was (or subsequently becomes) a British citizen.
If a parent was a British citizen at the time of the birth, then the child was born British. If a parent only becomes a British citizen after the child's birth, then the child can be registered as a British citizen, as long as this takes place before the child turns 18.