JasonL
I'm with Jim on this.
If the precedents you mention are what you included in a previous post ie volumetric concrete mixers, cranes, truck mounted cherry pickers, recovery trucks............. you will realise in each of those cases the operator is there with the vehicle.
I used to spec vehicles for purchase by North West Water, some had pto equipment like tippers, cranes/loaders, sludge tankers, gulley suckers - all of these had the operator in attendance.
I think, but will stand corrected, that leaving your car unattended with the engine running is an offence.
Same with a truck.