Really enjoyed Britannia and committed to watching all 9 episodes in quick succession. It didn't finish however! No questions were answered and it was more like a mid season cliffhanger. Feel a bit cheated! I can't see anywhere that there's going to be more. Does anyone know?!
I too thoroughly enjoyed Britannia, all nine episodes, and in response to your season 2 question I could find no mention of when it will appear other than 2018.
Mind you, there is one thing that "jarred" me, and that is the use of modern expressions in, what is suppose to be, an historical drama, with an added touch of mysticism.
I can forgive that one failing because, I believe, the makers want to sell this show to America and so some artistic license is necessary.
However, Britannia is nowhere near as bad as the failed "Robin Hood" series the BBC made a few years back. In that show you had British actors in a British historical drama constantly using American expressions, it was just awful. Mercifully, that series came to a quick and premature end, and hopefully, never to be endured again.