and that still doesn't change the fact that the Lambo and MGT could be charged with reckless driving, drag racing, etc. but most likely some form of negligence. The Mazda driver has an easy case in return.
The law in the US (not familiar with London) but here the law requires drivers to be careful when encountering anyone they meet on the road including passengers, other vehicles, and pedestrians. It's called duty of reasonable care.
So while the guy in the Mazda may receive a traffic violation (not likely though), he's going to have a great case in return against the Lambo owner, especially if he was injured. I would say he might be the one driving the Lambo next time. He likely won't get a violation even if the Lambo is charged with negligence or drag racing. So no, he isn't automatically going to be at fault if he turned out in front of the Lambo if it is proven he was racing.
The law looks at the driver's conduct and compares that with the conduct expected of a reasonable person. How would a reasonable, prudent person have behaved in the same or similar circumstances?....not likely the way he was driving.