I know people believe this but the UK does have a constitution, it's the sum of laws, treaties, conventions and the Common Law as interpreted by the judiciary over hundreds of years.
It is no less real than the American Constitution. What it doesn't have is a single written document (with amendments).
That makes it a bit harder to go read, don't you think?
It also lacks a fundamental feature the US Constitution has in being umambiguously final. No other law can supersede it. Interpreting an entire corpus of law as a 'constitution' is grossly inaequate.
The U.S. Constitution is far from unambiguous, and U.S. constitutional law can only be understood as a corpus of precedents which explain the meaning of terms which are open to interpretation, like 'due process' or 'freedom of speech.'
It is no less real than the American Constitution. What it doesn't have is a single written document (with amendments).