Right. That's the same logic for banning any device that attaches the child to an adult during those phases of the flight, including baby carriers. They imply in their advertising that it's not "yet" approved but in the decade or so this gizmo's been around, it hasn't happened.

On the car seat issue am i right in understanding that UK rear facing seats cannot be used on planes fullstop (due to the shoulder belt issue)? i'd presume this is why the UK airlines also only accept forward facing seats (which can be attached using the lapbelt)

Is this the cart before the horse or the other way around? Perhaps since Britain is smaller and more dependent on train and road travel, there is little demand for airplane-approved car seats? So they continue to have car seats that need shoulder belts to install? It's hard to say. Aren't there similar rules in Australia? Most of the car seats sold in N.America are FAA approved so it's not an issue there.