I would, regardless of the law. I don't believe you would be charged if you could demonstrate that they attacked you and you didn't do anything to provoke it (this is why its always good when threatened with a violent situation not to escalate or contribute to it with trash talk, because that kind of negates any self-defence argument).
If you actually killed someone and they were unarmed, you probably would end up having to try and persuade a jury that you used reasonable force, which, rightly or wrongly, would be a bit of a tough sell if you were armed and they weren't.
In Britain, if you are only allowed to kill someone if you have very good grounds to believe that either you or someone else is in danger of being killed (this applies to the police as well btw). Killing someone to protect property is not allowed.
Obviously, it would be really really difficult to prove you had reasonable grounds to feel you were under threat of death or serious injury if you were a 35 year old 16 stone bruiser armed with a knife and you killed an unarmed 12 year old burglar. At the very least you would end up having to explain it to a jury. You wouldn't get away with not being charged like you might if the boot was on the other foot...
This is a core concept in teaching about justification for the use of force in the United States.