Not my main field, but here's a few quotes from historical documents that might help out a bit:

"All the Britains, indeed, dye themselves with woad, which occasions a bluish color, and thereby have a more terrible appearance in fight. They wear their hair long, and have every part of their body shaved except their head and upper lip."

The Gallic Wars V, by Julius Caesar, translated by W. A. McDevitte and W. S. Bohn

"In stature [the British Queen Boudicca] was very tall, in appearance most terrifying, in the glance of her eye most fierce, and her voice was harsh. A great mass of the tawniest hair fell to her hips; around her neck was a large golden necklace, and she wore a tunic of diverse colors over which a thick mantle was fastened with a brooch. This was her invariable attire."

The Works of Dio Cassius,English tr. Earnest Crary [1904-1927] at sacred-texts.com, modernized by Dr. G

You might also find this article by the British Museum useful.