It's all a mess, and has been for years....
My grandfather worked for the Foreign Office/Diplomatic Corps and spent most of his working life in India. My mother was born in Calcutta, her sister in Delhi another sister in Lahore etc etc, all in civil hospitals, but with a "Diplomatic Birth Certificate"
My mother was schooled in England - boarding school - and on leaving went straight into the WRAF - it was 2nd world war time. She did time in England, India and Germany, during this time she met/married my father, also in the RAF
Move on 25 years, my father died young, after a few years Mum decides/is persuaded to go on a holiday to Yugoslavia.. but she needs a passport... so we do the paperwork, to discover she is not entitled to a British Passport - in her own right - but can have one if we send in copies of Dad's Birth/Death/Marriage certificates !!!!!
So her father can work for HMG, she can serve during the War, but she can't have a passport except she can as she was "lucky" to marry someone with a passport already !!!!
Madness, utter madness
:-/ :-/