Why did Winston Churchill commanded the Uk armed forces during WW2 ?

I'm not from the Uk, and I'm trying to understand the role of the monarchy in the united kingdom. I understand that the king is the head of the state & armed forces and the prime minister is the head of the government. So why did Churchill commanded the military? and I have seen so many TV doentaries about this and none of them even mention the king, I'm a little confused here and I'd like to know a little more about the subject. Please I would appreciate answer from people that really knows . Thank you very much