okmatewanker

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.