Tom C. - "What on earth does this mean? What personal experience? Is it just a kind of warm fuzzy feeling of "christian-ness" you get by walking around?"
What I mean is that if you live in Bradford or Southall, for example, you will quickly get the impression that most of the people are Hindu or Muslim, very differently, say, from Golders Green or High Barnet. In a village in the country, you might have some four or five different churches, e.g. CoE, Catholic, Methodist, Presbyterian and Baptist, and pretty soon you get to know which families frequent them (from time to time).
Here in Brazil, most people have been baptised into the RC Church, but not so many are weekly attenders (in the cities, at least), attendance being far more obvious and assiduous at the so-called Evangelical (cover-term for Protestant) Churches, and one meets with a fair number of Kardeckian Spiritists, yet signs of Afro-Syncretic beliefs (Candomble and Umbanda) are omnipresent and not mutually exclusive.
Unless someone has made a personal choice to convert to another religion, I think most people will answer a questionnaire with the religion in which they were raised, whether or not they are regular practitioners, and this in most cases is that of their parents and families.
This is the kind of thing I mean by personal experience here, and yes indeed, it is rather subjective, but then so are most people's personal beliefs, which is precisely why it is so difficult to quantify them.
In terms of statistics, I quote what is available, and, preferably, recent. I guess the Times, as a publisher, must be a very dubious source, but I have none better to hand.