Thanks for your pointless little gem, but cutting someone in half could never be construed as reasonable force.
There was a case recently, and I mean more recently than Tony Martin, where a shotgun was used. Another case, in Manchester, resulted in a machete-wielding burglar being stabbed to death, and the householder than killed him was not even prosecuted, and quite rightly.
Far from my comment being pointless, it is absolutely the entire point. The law says NOTHING about cutting in half, nothing about which weapons can or cannot be used, merely that force in self defence must not be grossly disproportionate,
in the circumstances.
It doesn't much matter to a deceased burglar, or indeed to prosecuting authorities, whether the deceased is cut in half by a sword or a shotgun, or indeed if they are cut in half or not. That is all pointless forum drivel. What matters is the force used, the degree of proportionality or otherwise, and the genuinely held belief of the householder as to the threat they faced.
In the case of that Manchester burglar, the householder faced a machete-wielding intruder, and is quite justified in fearing for his, or others, lives. He is not expected to exercise the level of judgement required to know, for instance, that stabbing the machete nut in one place will disable him, but in another will kill him. He's just genuinely in fear for his life and entitled to use force, whether knife, katana or shotgun to defend himself, up to and including deadly force, providing it's not disproportionate. Where that line is depends on circumstances. How many intruders? How many shotgun shells did he have? Where were the intruders? Was a warning shot feasible? Did he have time?
It's that entire accumulation of circumstances, given the panicked state of a householder, that determines what's grossly disproportionate and what isn't. If the only weapon available is some fancy sword, and a panicked householder swings it wildly, he is extremely unlikely to be expected to judge whether that swing will kill but not utterly bisect, or whether it leaves the intruder in bits.
If deadly force is reasonable, then it's reasonable and if a wild swing is reasonable, then it is.
On the other hand, if you lurk in a dark corner and let an unarmed intruder sneak past you, then cut him in half from behind, it's much less likely to be reasonable, because you effectively laid in wait. As for him being unarmed, what matters is what you genuinely believed, not what was actually the case. For instance, if he was "armed" with a non-firing replica, the threat posed was minimal but you weren't to know that.
There is no way to be certain in advance what actions, or weapons, are acceptable or not because rules on that don't exist. Guidelines do, but they illustrate the actual rule, which is the one I gave. All this rubbish about cutting in half is exactly that.