it's legal to undertake when the stationary traffic is moving slower/stopped due to a traffic light or junction. His manoeuvre, while not illegal, was probably initiated by a driver in your lane flashing/waving him on, which is illegal, and this crash is why it is illegal. In short, legally he is in the wrong and you are in the right, aside from the brakeless bit.
it's legal to undertake when the stationary traffic is moving slower/stopped due to a traffic light or junction. His manoeuvre, while not illegal, was probably initiated by a driver in your lane flashing/waving him on, which is illegal, and this crash is why it is illegal. In short, legally he is in the wrong and you are in the right, aside from the brakeless bit.