You can have a British car but only British people are Britons.
It gets more complicated in history as Britons referred to the Celtic people of Britain, as opposed to the Romans, then later the various other peoples who made up the rich tapestry of Middle Ages Britain: the Anglo-Saxons, the Vikings, etc. It's why the British Isles includes Ireland because the Insular Celts were once all Britons.
The Lady of the Lake, her arm clad in the purest shimmering samite, held aloft Excalibur from the bosom of the water, signifying by divine providence that I, Arthur, was to carry Excalibur. That is why I am your king.
Le royaume de Logres n'a jamais atteint les portes de mon pays et le fils de Pendragon n'a aucun allié dans mes terres. Accepte ta défaite et fait preuve d'humilité tant que je le demande courtoisement or me and my people shall teach you politeness by the edge of our...
Forget about that I'm a pacifist. But still, I'll pacifically reject your authority and the authority of whoever invested you with yours.