A true Brit should not eat swans as much as Jews and Muslims don't eat pigs.
And besides, British royals are Germans not Brits.
One's father's ancestry doesn't determine one's nationality. The majority of King Charles III's ancestry does appear to be German (about a quarter British, and a good helping of Polish), but Prince William is majority British.
Personally I don't even think where you were born has to matter, just where and how you spend your life.