Indeed. Luckily enough it never happened to me, even though I lived in London for 21 years, but it happened to a couple of friends. What's the law on shooting a mugger in the US?
How do you know hes only going to mug you? We have to trust the criminal? Who may have a knife or gun on you?
We only have to fear for our life or permanent bodily damage. Generally once confronted there is little to no chance to try to get away from the situation.
Surprise attacks allow for greater use of force as a response. Its called Disparity of force and it is an established legal principle used by the victim/target of a criminals assault to justify their actions.