When following this route, aircraft arrive at 90 degrees to the river Thames at Vauxhall Bridge, which is a compulsory VRP. The river is less than 300 metres or so in width at that point so a very sharp turn to starboard is needed to join heli-route H4 there, unless you fly a little bit further east and put a radius on the turn.
If that is the route this aircraft took, it would be possible that there was an overshoot over the opposite bank of the river in the turn. It'll be interesting to see if this is the route it took, and what the speed was.