As the website points out:
"In 2001, a highly leading question, "What is your religion?" was added.
By assuming that all participants held a religious belief, the question captured some kind of loose cultural affiliation, and as a result over 70% responded 'Christian', a far higher percentage than nearly every other significant survey or poll on religious belief in the past decade." In spite of vigorous efforts by the Humanist Association this same question will appear in the 2011 census.
So what the campaign is wanting people to do is really consider the nature of their own beliefs, irrespective of their parents, their upbringing, whether they might have been christened or baptised, and asking that if they do not have any personal faith/belief in any kind of deity they should tick the 'no religion' box. Statistics from this census will be used to decide all sorts of political/economic policies over the coming years and a skewed result about the extent of religious commitment would not be a good thing (why it matters here).