Immigration
My son was born in BMH Hong Kong when we were at Kai Tak. He has a Kowloon City registrars birth certificate (in English and Cantonese) and a British birth certificate issued by the appropriate military authority (I can't remember which). At census times we just ticked the box as being born in the UK, there being no other appropriate box. You also should have a British birth certificate issued by the appropriate military authority at wherever your parents were serving . My son is now a SNCO in the RAF and I don't think that the government would like him being deported and discussing his current service with representatives of the Peoples Liberation Army. I think that the Windrush thing is a bit of a red herring in this respect.