Thanks secret.simon for the very thorough reply. Not the answer we were hoping for, but I appreciate your help!

My partner was born in the US in 1974. Her father was born in New Zealand in 1936. Her father's father was born in the UK circa 1905. Her father, now deceased, had a UK passport. We don't know if he was a citizen by descent (via his father) or otherwise than by descent (by virtue of being born in ...