ISIS has formally declared itself the arbiter of all humanity and will enact the will of Allah at all costs. It cares not for nation states as we know them, it cares not for the law, or for democracy. Allah is the state, the law and knowledge on all things.
You are a fool for thinking there is negotiations with such dogmatism. Those who think along a narrow path of dogma express themselves along the narrow edge of the sword. Negotiations are off the table. The question is if British military intervention is the correct form of policy.