And here finally is the answer: In his 5th commentary Caesar describes the Britons (ch. 12), the lay of the land (ch. 13), and the more civilized Kentians (ch. 14) compared with the people living in the inner parts of the Isle. In the 14th chapter he writes: "Omnes vero se Britanni vitro inficiunt, quod caeruleum efficit colorem, atque hoc horribiliores sunt in pugna aspectu" Indded all Britons apply woad upon themselves, which gives a blue colour, and so they are pretty horrible to look at in combat [my translation]. About fighting naked I didn't find a reference in Caesar's commentaries, neither here nor in the report about his first invasion (4, 20-26).
About the rubbing off of woad colour: I'm not so sure about it anymore; woad was the cheapest dye in Germany, too, but I never heard of peasants with blue hide over here - but God knows best. So let Caesar's observations stand as they do.