It seems to me that when people in the UK aren't religious they tend to say they're CofE.
Perhaps they used to, not sure if thats still true. It used to be the polite answer
Theres a difference between religion and faith and you can be part of one and not have the other. Its far too nebulous a question to be able to have everyone answer the question by ticking yes or no. And over different decades when the census is taken the questions and answers have different meanings. There are times and places where you could ask the question where people would describe themselves as Christian regardless of whether they actively practice any kind of religious observance. Currently though atheism is quite fashionable, or rather expressing atheism is, so people are more likely define themselves as positively atheist rather than passively part of a christian tradition.
But where religion has a different context the answer to the question might be different. In the west of scotland you might define yourself as catholic or protestant on the basis of which football team you support, even if you haven't stepped inside a church for all your adult life, and obviously in NI your political position is mixed up with religious identity, and in that instance you may well feel that both your religion and your politics are something you've been born into rather than chosen or objectively assessed.
The US is quite interesting in this respect, statistically a very christian country, but genetically and socially not very different to us. In the US church-going is very much the social norm, and its the norm whether you have any firmly held belief or not. During his election campaign Obama was hit with one of those 'When did you stop beating your wife' questions and it was to do with the pastor of the church he attended and some quite radical views they held. The reality was Obama would have had no idea what views the pastor held as he wasn't actually going to church. He'd done what a lot of people do - say (when asked) that he attends a church that has a huge congregation - so big that way nobody notices that your not actually there. And you say/do that out politeness, its more polite to lie about your religious commitments than to say you don't have any.