There can be few more colorful people than the ancient Britons. With their trademark spiky limed hair and magnificently droopy moustaches and painted with woad battle-markings they were as colorful and they were ferocious.
The British, who still favored the chariots as a key part of their armies, struck fear into the hearts of the Roman invaders, led as they were by the dark and terrifying Druidic cult. There is a famous account of Romans not wanting to leave their boat to tackle the frenzied British and their Druid leaders on the shores of Britain. It took one of the standard bearers to throw his standard ashore and thus driving his companions to go and protect it to break the spell!
It's not all about Druids though - there were several highly capable British leaders who resisted the imposed rule of Rome such as Caractacus and the one-and-only Boadicea!