Well, it's called that, but it might be more accurate to say that it is written in the form of hundreds of years of legislation, treaties, and judicial decision

Well, it's called that, but it might be more accurate to say that it is written in the form of hundreds of years of legislation, treaties, and judicial decisions, just not compiled into a single document. See https://en.wikipedia.org/wiki/Constitution_of_the_United_Kingdom https://en.wikipedia.org/wiki/Constitution_of_the_United_Kin....