TalkinPeace
The OP is not talking about RE, the OP is about compulsory prayer.
as I've said at our school no one is policing whether they students actually join in.
That's not the same in all schools though, I understand your primary concern being your own children but what about other children?
Vast majority of school assemblies (even in some Church schools) are an opportunity for the whole community to come together. There are usually some notices, quite often a 'moral' story, and sometimes (where the school is lucky enough to have a musician on the staff) a chance to sing. Very often, there is the opportunity to celebrate / share someone's achievement or good news. I think this is a good thing. I can't see what's not to like.
And that is not what is being asked by the OP.
Worship is compulsory, Christian worship, not Jewish, Muslim or any other. Assemblies are not compulsory. In fact the organisations who are trying to remove compulsory worship want assemblies to continue.
In some schools, there is a prayer, which each and every person in the room is able to choose if they participate in
Only if the school is breaking the law, which many do. But why put teachers in the position of either breaking the law or forcing children to pray?