everbody's dead
In a situation where the helicopter hits the ground/water and autorotation hasn’t sufficiently slowed the descent/impact rate its going to make little difference if it’s a lap strap or a 3,4 or 5 point harness or if your seat is full back length or just up to lumbar region. All your rules go out the window, literally, because everybody is dead.
The seats are bucket type, bolted to the main deck and the skids may have some shocks to reduce ground resonance. The seats and harnesses on small helicopters, civilian or parapublic are a minimum requirement to comply with safety regulations and as has been said a design compromise. Weight saving and product cost is always going to be more of a priority than safety with operators. Compare the engineering in a Martin Baker crash resistant seat and crumple zones on combat helicopter hulls. The effort that goes into ensuring crew survivability is of another magnitude.
True, when your family of sightseers come cantering hesitantly in a stooped position to embark the helicopter, the noise and downwash & confusion of this new environment makes it difficult for a novice to figure out a 4 point harness, which means that someone has to show them. Twist to release four point seems to make people feel more secure than some airline style lap buckle harness.
I wonder if they made it mandatory to have crash resistant seats and 5 point harnesses, if they would allow more heliports to open in London. Kind of like a compromise?