Yes, you are overreacting (although I do think the school should have provided a lot moe information), I've been involved in schools in Scotland and have been involved with taking groups to Abernethy Outdoor Centre (and similar places).

First, I think that UK law says that schools must provide a "collective act of worship", I don't see why, if it is the schools responsibility to do that, it should be shelved duringa school activity.

Secondly, (state) schools in Scotland tend to link more with local faith groups than Englsih schools do. I worked for a church in Scotland and the school wanted me to meet the OFSTED inspectors to show the importance of the relationship. I now work for a church in England, schools do the opposite, they don't want OFSTED to know about any links with faith groups.

Thirdly, all staff at Abernethy will be CRB checked (enhanced) and generally the group leaders (in this case the school staff) are usually the ones who look after the children outside of the amin activities.