And the statistical expectation is, I assume, what The Humanists were complaining about when they challenged the wording of the census as the last one implied 70% (rounded, can't recall the actual figure) of the population were Christian, whereas that encompassed everything from apathy to starkly non religious.

People tended to put Christian meaning common or garden British, it appears. Hence I was challenged recently why I don't go to the local church. My reply was to ask why they don't assume I go to a Mosque, synagogue etc.

So your statistical expectation seems to ring true, although if only it didn't....