I think that was pretty decent job of explaining things Adam.
The act of maintaining possession of the "weapon" is rather frequently being treated a de facto evidence that you committed a premeditated crime. In the UK there doesn't appear to be any way to be prepared in advance to defend yourself with lethal force without getting caught in this catch 22.
I will close with the disclaimer that I have not done a lot of research on the matter and would be very pleased (for those in the UK) if I am completely off base.