As mentioned above in Phil's link, probably British Canoeing guide qualifications, presumably raft type might also cover tubes. These normally also require things like Whitewater Safety and Rescue and specific first aid as separate qualifications.
What grade is the planned river (may affect level of quali and insurance)? Are there proper access points without crossing private land, where a commercial group won't impact on/upset locals? Not a problem in all areas, but there are a few places where this can be a sensitive subject. Is the river regularly fished (similar reasons)? What gear will you supply (buoyancy aids, helmets, wetsuits)? Guide/customer ratio, which may depend on grade of river.