Very often in London, you have to creep forward, to get a better view and due to bad traffic. I would say that although the Aventador has right of way, the fact that he/she was going so fast that he/she couldnt brake in a 30mph zone suggests he/she was going way too fast for the conditions and will have a significant proportion of blame to accept, perhaps more than the Mazda driver. Driving without due care comes to mind. Visibility is good, road surface is not wet, so no reason for a modern supercar being unable to perform an emergency braking manoeuvre.
The noise from the Lambo as well as the speed at which the car impacts, suggests speeding.
Edit: The brake lights dont come on till the second impact. Distracted Lambo driver, I think.