The aerial views video (on the BBC news site) show me a roofstructure of the pub that has exactly the same colour and appearance as the (blacktop) area around the pub. Although I'm not familiar with the immediate surroundings of this area (and ofcourse I don't know how the street looked at that time of night), I find it totally believable that the crew (under stress of the moment, and in night conditions) had a hard time to distinguish rooftop from blacktop around it. What I mean to say is...it may not have been the intention to land it on the rooftop; in fact, maybe they didn't initially realize they were aiming for a rooftop.
Auto's in the dark are no easy thing to do. Mr. Miller confirmed that the helicopter made a vertical descend to 'impact'.
I would make it top priority in my autorotation to get as little forward speed as possible...especially in this area where there is not much space (if any) for a running landing. They may have ended their flare at an altitude a little higher than originally anticipated (depth perception in the dark/goggles is not an easy feat.)
Nr might have been traded off during the final (vertical) descend while trying to control the RoD.