Where is that written as law? In the UK, nowhere. It devolves into overpaid legal eagles arguing at enormous expense over a constitution which neither side can put their finger upon. Because it is not encoded and enshrined into law in any meaningful way.
The so-called British Constitution does not exist in any meaningful way because it simply does not exist at all. All that actually exists is precedent, which is subject to wild interpretation, legal argument, which is subject to the relative ability of the legals on either side, and appointed judges whose rulings are often called into question.
Ideally, the UK claims to have a constitution. Nobody has ever seen it or read it.
If you disagree, feel free to point out the UK constitution.
Constitutions don't have to be codified to be called a constitution. We certainly do have one, problem is it's wrote on the back of several envelopes, a napkin from 300 years ago and the medieval equivalent of post-it notes!