DH has an elderly relative by marriage, and has been appointed their executor.
The relative was born in France, their mother was French, and their father British having served during WWI, and stayed in 1918.
When the relative was born, their birth was registered in the village, but neither the British Embassy in Paris, High Commission or any other British authority were informed.
The family left France, the relative had no siblings or children, in 1939, and they have lived permanently in the UK ever since. At the age of 21 they were serving in the British armed forces and so took up British nationality and have had a British passport ever since. Their parents returned to France, to the same village as before the war and are both buried there.
Due to this strange set of circumstances, their birth is not registered with the GRO, but their 2 marriages and death of their spouse are. The marriages and death are officially unknown in France, even though the authorities opened the parents' grave to bury their spouse's ashes. To the French authorities they are officially single.
So, given that due to the relative time they spend in France and the UK, they are likely to die in the UK.
With no record of their birth, how can their death be registered in the UK?