Either way it's still homicide, but in the case of self defense it would be justifiable homicide. Sorry to hear that you changed your mind back GD. Although Britain may have an entirely different situation than the U.S., I still do not think banning guns has done anything to help the crime there, and in the U.S. it often has a negative impact.
I've noticed how people call for the banning of guns or stricter gun control whenever something terrible happens involving guns. I know a lot of people that go to Virginia Tech (though I didn't know any who where killed). Last year the parents of those who were killed went to the Virginia legislature and pleaded with them to ban guns in Virginia. This is usually the first reaction to such a crime, it's emotional in my opinion. The facts are we have very little to fear from law abiding citizens carrying firearms, and in the rare cases where you get a psycho like Choi, armed citizens are the first line of defense, and in the past it has saved lives. At VT you had an entire building that was at the mercy of one man because guns were banned there. He ignored that ban, like most criminals would. In addition to this, there were several things that could have been done to warn people of Choi, it may have helped, but a guy like that can only be dealt with one way.
In the U.S. there are currently 1700 people for every 1 police officer on duty, it is impossible for them to protect everybody. You have to protect yourself. Where the government needs to do a better job is make sure these criminals, when they are convicted, go to jail for a very long time. The problem with crime here is not guns, it's the revolving door method of our prison systems. An average of 5 years for a rapist, 7 for murder. I say let people carry their guns for lawful purposes, if somebody abuses that right, the citizens have the immediate means to defend themselves, and that criminal (if they are still alive), should go to jail for good.