But eyewitnesses also reported it flying low along the river, and the AAIB report says it fell from 1000 feet.
If the rotors were stopped, it could in principle "tumble", but in the dark, can a single witness be trusted to say whether it was spinning around the rotor axis, "tumbling", changing in pitch or whatever? If it was tumbling, then it might be interesting, as that requires the rotor to have been stopped before that tumbling began.
The AAIB is clear that it suffered extreme deceleration/was upright on impact. I don't imagine that its orientation during this fast descent is very important for understanding the causes.