> If there's just one British constitution and everyone agrees on it, then it seems like writing it down would be pretty trivial.

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.

But those who do not agree with it do not have any other custom or tradition which they claim is actually the constitution. Original claim stands on the argument that if there was really a single constitution, it would be trivial to write it down, not that everyone needs to agree on it, which is not a viable criteria anyway to test if something is constitution or not.

The majority would suffice.

Leaving aside whether there is only one constitution, when has writing any governing document ever been trivial? Literally, humanity has struggled and continues to struggle to write down and effectively communicate even documents of much lesser importance, API documentation or text books for example. I'm sorry but I don't see how this point can be made seriously.