I think the law is about right. If most children in a school are Christian (non practising) then the act of collective worship is Christian and the athiests and children who don't want to participate don't join in. If it is a manly Muslim or school then the act of collective worship reflects that by getting an exemption. Many schools are multi cultural so they just pray to God, but no God in particular.
So the school reflects the majority, but makes provision for the minority.
I don't think the same attention should be paid to all religions because we are not encouraging children to choose any religion, but simply to have a vague understanding of spirituality and a cultural, non practising understanding of religion. Culturally, we live in the West, and the religion that has most influenced our country is Christianity. It is important for understanding politics, history, art, philosophy etc of the West, and we are Western! Secondary to that is the other Abrahamic religions (as they are a significant minority and their religions are intertwined with the development of our country), then the Eastern religions, and then the religions of tribal Amazonians etc.
It could be that the Amazonian tribe are right, and everyone else is wrong. Morally, the tribe could be right and everyone else could be wrong. The religion of the tribe could be the right one for your child.
But we cannot teach it in school, because we need children to understand most about the belief system that influenced their country, and then other major world beliefs, and then the minority beliefs. It would be very ignorant to have been educated in Britain and not the the nativity story; it is in the new testament and the koran and is known to at least half of the population of the world! (30% are Christian and 20% muslim)
As for people who think we do too much of other religions and not enough indepth Christianity at school. You can learn more at home! it's not like having to teach physics; anyone can read their child the parables. The Percy the Park keeper writer has done books of them which are very nice.
Just my opinion, but I have an extended family of Christians, Athiests and Muslims and I a think the vague worship done in schools is the best compromise. If people want more religion, they can teach it at home. But if we get rid of all religion, then we end up like the US, with fundamentalist (learned from the home) children who have no knowledge of other religions, because the school doesn't cover it.