Britan is named after the Britons. If you follow that to the logical end, it means "the painted people", like the Picti in Latin.
France is named for the Franks. Which might mean anything from the "francisca" which is a throwing axe in Latin, to some proto-germanic word which means fierce.
Wales comes from the proto-germanic word "Walha" which means stranger. This is an insult as the word "Cymru" means friend.
Still out there!