@CurlewKate Christians often use “well, you can opt out” without considering that this can mean a child having to miss assembly, and therefore a significant element of the school day. In a state funded non denominational school.
"A significant element of the school day"
How can it be "significant" when apparently, they are being taught stuff you don't want them to learn ? At schools I know about, certain children who were "opted out" of this sat together and had a person of their own faith come in to teach them. Or they were supervised by a mainstream teacher while they used material provided by their own parents.
Parents do have agency in this.