Hi I am a Pilot as well as a Drone user, the flying rule states not to fly below 400ft unless approaching to land or takeoff. One exception is training for engine failure (single engine aircraft). where you select a field and approach but again the 400ft rule should not be broken.
So this Plane was breaking CAA rules. This rule also applies to Heli's.
This is the reason why we are not supposed to be flying above 400ft....
About the same here in the states but, shockingly, there are manned pilots that ignore the rules too. Go figure. I've been buzzed by helos under 150 feet AGL.