You are being over cautious, which is understandable when you don't know what sort of trips they are talking about, but this is why you need to go in and ask them.
To you, there is nowhere they could possibly go because the local shops etc are 20 minutes away, but you aren't trying to deliver the curriculum in the most stimulating way possible, so you won't have the same ideas as the teachers.
AbigailS has provided some good suggestions of what the consent could be used for, none of which require the shops or library. My children have done all of those things at primary school. There are endless things that the school could do with your child, like going to a local field, or pond to see frogspawn, or visiting another school site. They will ask for permission for things like swimming or whatever, and they will have risk assessments for every single time they go out of the school gates. They will probably also ask for parent helpers, so will have an opportunity to go along and see how they manage things. At my dc's school, they ask for enough adult helpers so that there are four children to each adult, then that adult becomes responsible for keeping an eye on ther designated children to make sure they don't wander off or come to any harm.
It's not easy taking large numbers of children out, I know because I do it. We don't do it just for the sake of worrying parents, we do it because we really think the children will learn from it and enjoy it. It's fine for you to be a bit concerned, but it is insulting that you don't trust the school staff. They genuinely want the best for your children.