Contrary to popular belief, we do have a written constitution - but it's buried in thousands of official and legal documents written since, oooh, about 1215...
(If anyone doeswant to investigate this sort of thing, Peter Hennessey is amongst the best on constitutional conventions. Try The Hidden Wiring: Unearthing the British Constitution (1995) and The Prime Minister: The Office and Its Holders since 1945 (2000). We now return you to the banter...)