David Cameron's right, regardless of the beliefs or lack of beliefs among the UK population. You have an official state religion (Church of England) whose head is also your head of State (that decrepit, presumably less slutty Paris Hilton called Queen Elizabeth the second), plus you live in a monarchy where the divine right to rule is given by God to the royal family by Fate through kinship or some other BS.
You can pretend to being a secular society once you get rid of the monarchy. Until then, the entire Commonwealth is composed of Christian nations (unless the monarch starts or adopts a new state religion that is not part of christianity; or gets rid of an official religion altogether, though that divine right to rule (even just as a figurehead) sort of ties at least theism with the whole Commonwealth).