//Citizenship by double descent
British nationality law allows claims to come from a UK-born grandparent (known as double descent).
If you were born after 1 January 1983, you may have a claim to British citizenship if:
You or a parent were born in a former British colony
Your grandmother was born in the UK, and your relevant parent was registered as a child before 1983
You have a UK-born grandfather who was in Crown service at the time of your parent’s birth//
https://www.google.ca/amp/s/www.biznews.com/global-citizen/2017/10/17/claim-british-citizenship/amp