And it is of course a voluntary question meaning those answering do so of their own volition and some intent.
I suspect most people merely answer it a it is the next question on the form and there are many questions to work your way through.
The portrait painted by Humanist UK of people Jockeyed into answering and in a certain way is therefore inaccurate.
Go take that up with HumanistsUK (and at least get their name right). I am not HumanistsUK, I do not speak for HumanistsUK and I am not a member of HumanistsUK.
However it is clear that the census approach is the one that provides the very highest levels of christianity, not least due to the leading question. There is also the issue that it deliberately assesses broad 'affiliation' however nominal. It does not assess belief or activity (the latter two always reduce the number claiming to be christian dramatically, and indeed other religions somewhat).
If fact there was a proposal that the 2011 census should also include philosophical beliefs (as you've suggested), but it was rejected because it would require the question to be about religious
belief or philosophical belief and this was considered to skew the results - a reduction in those indicating a religious affiliation (however nominal), rather than having to indicate a religious
belief.
Now I understand from a census perfective why they didn't want to make the change as one of the purposes of the census is to look at changes over time and altering questions can make that difficult. But the point remains that the leading question and the fact that the question is about affiliation however nominal in combination will provide data that gives much greater numbers of christians (in particular) that approaches that use non-leading questions and more nuanced questions.
Now seeing as one of the purposes of the census is to support public service provision, funding and policy lumping together:
A) a census christian who merely ticked that box because they were christened and as an adult neither believes in any of the key tenets of christianity, indeed may not even belief in god and the last thing they'd ever do is attend a religious service (except through invitation for a wedding etc), and for whom religion is an irrelevance but who may abhor the attitudes of many religions towards women, gay people etc.
with
B) a 100% believing and committed evangelical christian whose whole life revolves around a church and its activities and attendance and who seeks out a family and friends who largely do the same.
Chalk and cheese in the extreme. But in the eyes of the census (and the policy implications that flow from it) they are both the same.