IANAL, but my brief understanding is: you may only use force to defend yourself against force, and you may only use lethal force to defend yourself against lethal force. Self-defense is a valid defense against certain crimes (which I understand is not quite the same thing as being a right, though I can't say I really see the difference); there is no "castle doctrine". When the famous Tony Martin case happened (a farmer shot and killed some burglars (few people have guns in the UK, but farmers are an exception, and relatively commonly own (licensed) shotguns on the grounds of needing to shoot vermin)), I remember a local police officer's comment being "if they were shot in the front he'll get off, but if they were shot in the back he's going to jail".
I think this is pretty similar to the rest of the western world, and certainly to the legal frameworks in much of the US. The interpretation is probably very different, though. So is the legality of carrying weapons in anticipation of an assault...
