If it's dinghies, probably one of these:
http://www.wayfarer.org.uk/index.php...=25&Itemid=173
I notice all your pics are of 6+ M boats. If you are teaching powerboating, any or all could be valid. You probably want a selection anyway to give a good spread of learning (e.g. SIB / RIB/ Dory , big / small, single /twin, wheel / tiller, planing / displacement.......
If for rescue / teaching at a sailing centre, probably nothing bigger than 5.5m or your fuel bills will cripple you. I *think* the RYA has a reccommended boat - probably best to give them a call.
I've taught Oppies from a canoe in the dim & distant past.......