"Constitution" is just a fancy word that makes Americans feel like they were the first and best at something.

You can call a core foundational set of laws whatever you want, and you can have something called a "Constitution" that leaves lots of institutional and procedural and justicial gaps, in addition to leaving 90% of its denizens without a full complement of rights.

A br*t I know always talks about how it's actually really good that their's no formal constitution because the government can just shift to meet the current age. Like, my guy, you have human shit in your river, what the fuck are you talking about?

the brits dont even have one!

they'll tell you that they do have one, but they just... prefer not to write it down:

The constitution of the United Kingdom comprises the written and unwritten arrangements that establish the United Kingdom of Great Britain and Northern Ireland as a political body. Unlike in most countries, no official attempt has been made to codify such arrangements into a single document, thus it is known as an uncodified constitution.

they're quite committed to the bit, even going so far as having a whole thing they call "UK constitutional law" (replete with varieties of court dress).