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_Kin....

I get the feeling the Brits like to not have too many written rules. Including s constitution. Personally I think it is detrimental to them.

Detrimental to whom? The upper classes love it: just make up stuff as you go along, depending on your needs.

By that criteria all the countries in the world have had constitutions from the beginning of time :)

We do have this (I know it's not the same thing exactly): https://en.wikipedia.org/wiki/Magna_Carta