70something % of the population considers themselves Christian in some form.
Its only really 70% though because it what people default to because of things like the OP is talking about. I don't think most people really think about religion, but when they are asked in a sense they think, "Well, I was christened", or "My primary school was CoE", or "I guess I kind of believe in something" and because cultural default is Christianity that is the box they tick, because they don't think are are not bothered. Many of these people may not believe in God or if they "believe in something", that does not mean they are Christain, its just something they haven't thought about or don't think about much and just plump for the easy answer because they are not bothered. They may have some vague believe in a higher power this does not make the christian. A better question(s) might be:
1. Do you believe that Jesus Christ was the son of God, and he was sent to earth and he died to cleanse humanity of all our sins, that he rose again then ascended to heaven. That this shows God love for humanity as he sacrificed his son for humanity.
(This is my understanding of Christianity please correct if I am wrong)
Then a second question:
2. Do you believe in some form of higher being, or God, that there is someone or thing "above" us in some way?
I strongly suspect you would get a lot less than 70%. answering yes to #1.
Religious freedom is very important, and for many it offers great comfort, direction and community. In the OPs case he should just go and treat it as one of those going to see the kids things, but it is not right that the state should be funding religious schools and that if no one asks send them to a Christian church as default. It should be an after hours club if you are interested, that not to say schools should not teach about different religions, more teaching will occur to christianity due to the nature of history but this is different to being taken to church by the school.
Its also very sad that moral philosophy in frequently only taught within the context of religion, because it makes people equate morality with religion. Its not helpful for people to believe that the only way to morals are through religion, in fact personally I would say religion can (not does, can) hinder moral philosophy for some people due to immutable facts within their faith. This is definitely not the case for all religious people as many are very open to debate. The problem lies when a society at large associates morality and religion too closely. This can be seen by the fact that the church has some default seats in the house of Lords, primarily to offer moral guidance. This is not to say that it is a bad idea to have some seats reserved for those whose main concern when considering legislation should be that of morality and society; but it is bad when that view point comes solely from that of a religious perspective, and only one religious perspective.