Oh and CH definitely isn't for posh kids! The idea is to take kids with academic / musical potential from difficult backgrounds (family breakdown, abuse, parents with mental / pysical illness) and give them a good education away from their challenging family environment - very similar to what the proposed Lambeth school in Sussex is intending to do.
I don't think their backgrounds have to be all that difficult. A friend of mine went free of charge because his widowed mother was on a low income - she was a perfectly respectable widow providing a good home for her children. My friend wasn't particularly happy there and even more annoyed at having been sent when his brother, who had failed the CH entrance exam, did better than him at the local grammar school.