Yes ShyTorque but we don't know where he was when he aborted his planed route to divert do we? We also know the cloud base at the impact point was around 600' since the witnesses said the top of the building was shrouded in mist so would he have not chosen to use the most easily navigable route? But as you say the AAIB will know his exact route.
Chosen the route? No, because that's not always how things work. Entry to the airspace is requested by the pilot (and in this case, the destination), and ATC give a clearance, which includes the route and altitude to be flown.