Seems like you and I actually agree, and 3point14 is mistaken about where the UK draws the line.
What if a householder happened to have gun and ammunition ready to hand for some lawful purpose? Would their use against a knife-wielding invader then be viewed as "proportionate"?
What if the householder were elderly, or infirm, or otherwise physically disadvantaged? What if they were able to put a locked door between them and their assailant, and in the time taken for the assailant to break through they get their gun and use it? Would that be viewed as "proportionate"?