I think that proportional reactive force is acceptable so if you are threatened with lethal force IE with a knife and honestly believe your life is threatened then use of a weapon could be within the law, where it could get sticky is in proving you weren’t carrying the item to use as a weapon. I believe the scenario where people have picked up something to use as a weapon for self defence is called incidental or instantaneous arming.
As you may have noticed I have been in a scuffle or two, most if not all of those times I have had some sort of knife on me as they are always on my belt or in my pocket unless I make the conscious decision to leave them at home eg going somewhere posh for a meal
Not once have I ever thought about using one as a weapon and never will, even when I have had one pulled on me I have dealt with it by saying exactly where that is going unless he does one sharpish while eyeing up "useful" objects close to hand, knives are pretty useless as weapons so unless the person holding it has a lot of skill and experience they are not hard to beat
Twice I have been in that situation the first time the scrote decided it was a better idea to leg it and the second time I took a screwdriver off a woman that was intent in shoving it through my face and asked the boyfriend if he would like to try it, I honestly have nothing but contempt for those sort of people