From my perspective if you're charged then it's not considered OK - and not permitted. If the police charge you, then they've normally got a reasonable expectation that the law is on their side. Sure, you may get a good sympathetic judge, but in general if you're charged - you've broken the law to start with.
But to clarify - (and I could be wrong here), but my understanding was that you are not allowed to have any item placed for the purpose of self defense (that could cause GBH or death) - but you can use an item if it 'happens' to be there for the purpose provided it wasn't first intended.
ie:
a) if I purchase a baseball bat and have it next to my bead for the purpose of defending myself from a home invader, and they attack me - I'm in trouble because it was premeditated and will have to fight a charge in court.
b) if I purchase a baseball bat because I play baseball, and during a home invasion I happen to have my sport gear including baseball bat near me, and I use that, then there's a much greater chance I won't be charged because it wasn't premeditated.
c) If I have a bedside lamp next to my bed, and I grab that to defend myself and hit the robber in the head, again, I may be OK.
d) But if I have another object that's not normal for any other purpose than self defense, I'm up the creek.
e) If I have one of those 'self defense pens' and use that for self defense - premeditated, I'm in trouble, but
f) If I have a normal pen and use that I'm OK....
etc...
As such, this tells me that we are not allowed to have an item for self defense purposes. Yes - there are ways around this. (loop holes) such as taking up baseball

- but for the general population - the law basically denies anyone the basic right to have any item in the house there for the purpose of self defense, and you have to hope that the 'circumstances' that the home invasion takes place is one where it falls in your favour (or your clever enough to know this to start with and prepare and know the right answers when asked by the police, etc).
Until this concept is changed within our laws we can't expect anything else to follow suit. This is the root underlying problem that needs to be targeted. The general human right to be able to prepare to defend ones self and ones family that we have been robbed of.