I have read and re-read the posts and prelim AAIB report regarding this tragic accident and after the past few days of comments am hesitant to offer my thoughts amongst the helicopter fraternity for fear of offending anyone.
But , without prejudice, I offer my idea of a possible reason for the flight ending up where it did.
When passing abeam the London Eye, and looking up ahead, PB seeing a gap up ahead,mis-identified the Chelsea Railway Bridge for Vauxhall. Having been cleared to hold between Vauxhall and Westminster, he headed straight down towards Chelsea R/Bridge thinking he was on track for Vauxhall. As he reached the river he went straight into the holding pattern which he would presumably fly visually with reference to the river, routing back towards the next bridge up the river which he would think would be Lambeth Bridge, but in reality was Vauxhall. .
I agree this is only speculation, but just do not think that someone as experienced as PB obviously was,would make that right turn unless HE was certain he knew where he was..... and with the hostile conditions prevailing , it seems like it could have been a very easy mistake to make.
The flaw in my theory is that I presume his GPS would be showing something different for the desired track to Vauxhall , but this is the only idea I can see for his actions.
Mmm , I know...wait for the AAIB report!