Pretty sure you'll be in displacement mode at 6.7mph Mike. I've never watched the transition into planing on the GPS but would guess we do not plane (I'm talking 2 up now) until 10mph or perhaps a bit more. It's very noticeable as the boat lifts the bow sharply until it climbs the hump and levels out.
There are youtube videos of 3m inflatables and 4hp outboards planing but I bet you have to be one up with minimum weight and the inflatable nicely blown up. I had a 3.5hp Suzuki on mine last year and that certainly wouldn't get it on the plane.
Here is an image of the kids digging an uneconomic hole in the water just prior to planing speed. I guess they're doing about 8mph.
Photos are interesting and worth taking. We have a 4m grp boat as well and usually take both out together on the sea which gives a great chance to photograph how the they look in action.
BTW really enjoyed your Sharpness and Bristol Harbour reports.... might be tempted to do one on our activities.
A job move nearly brought us to your area last year but reluctance to move the kids at a crucial stage of life/schooling meant we didn't take it up. Access to salty water compared with where we are now was appealing though.