Most kids in the UK don't have religious parents these days
That's true, but it doesn't mean that religion has no role in shaping conventional UK views on nudity. Attitudes are shaped over many years, and several generations, my parents generation were somewhat religious, and that was shaped by their parents, who were more religious (or at least felt that they had to pay lip service). Where we are now is a watering down of the original religious feelings (eg, bikinis are considered acceptable these days), but there's still some remaining influence.
(And of course there's still bishops in the House of Lords, so religion is still part of our government).