Pre-emption and Self Defence

The British are currently debating how much force can be used by someone to defend their home from invaders. Under a private member's bill, people would be allowed to use "reasonable force" but not "grossly disproportionate force." Of course the distinction is not always obvious, as was pointed out in this Times editorial:

H]ow much force is it reasonable to use against an intruder whom one has no reason to believe is armed or is a threat, but of whom nevertheless one is terribly afraid because he could be armed or violent? It isn't, in other words, self-defense because nobody is attacking you—but it certainly feels like it. [link provided in Slate]

This sounds just like the discussions on the Becker (here)-Posner (here) Blog about the economics of pre-emptive war.