Totally agree. However, to be fair, we are talking about the GCSE syllabus- where students are taught about religions/comparative religions- and not about teaching 'a religion' per se. Quite a different animal.
The point is that such a syllabus should also include 'non religious' options of developing a moral framework- which actually get very close- as the basic principples come back, again and again- because they make human common sense. The old A'Level was called 'religious education and ethics' - so could easily include the above.
In many areas though, parents have little choice- the local school is the only school and CofE- even if they are not religious, or Hindu or Muslims, as was the case in the village where I lived when our 2 were little. Was I supposed to take them away from their friends to bus them into town or to another school far away (the next villages all had CofE schools).
In the last town where I taught- there was a 'choice' of 3 secondary schools and 6th Form. A private Grammar school, a Catholic school, and a Comprehensive school. The first was of course beyond the means of a large %, the second was perceived as 'better' than the 3rd- but only a small % of those who attended were practising Catholics- many had grand-parents who were, once, and who were baptised out of tradition more than religious belief. The 3rd school 'suffered' from creaming from the other 2, and was therefore not comprehensive at all. The idea of parental 'choice' is not always a real 'choice' at all- and comes with a price. My GC are in a CofE school, and the religious content is quite prevalent- they are definitely taught religion rather than 'about religion'- especially during Advent- but throughout the year.