Of course I agree that there is no written constitution, I said as much above. But there _is_ a constitution. Constitutions do not have to be written, unless you think otherwise?
It makes the constitution some abstract thing, rather than a very factual set of rules, which you want the constitution to be.
Honestly, a country can have a universally recognized and respected written constitution without "everyone agree[ing] on it." See United States of America, Constitution Of.