Rear end collision with no damage to the front of the leading vehicle: why is the rear driver almost always liable?

Recently I was involved in an accident where setting off from the traffic lights the car in front suddenly braked and due the short gap between mine and his car, my car hit the rear of his car. This all happened within 10 seconds of lights going green so we had around 5mph speed but due to the compact traffic I had about 1 or 2 metres of space between us. He told me that some car cut in front of him and that's why he braked but I feel he could have simply just slowed down. I'm caught in a middle of something that I couldn't have avoided because this happened at the traffic lights and the space between cars in that situation is normally not more than 1 or 2 metres. My insurer is now treating this as my fault and so does the other guy's insurer, all because I hit him from the back! I don't think this is just and not everyone hitting others from the back could be at fault as the insurance companies say. How can I take this up or who do I need to take this up against? Do i stand any chance of claiming my damages from anyone?