You didn't read the article. The BADGUY had the crowbar. The GOODGUY had the knife.
It doesn't mater what is used firearm crowbar knife fists. If you stop the threat and then continue to beat or shoot the person to death. That is unreasonable force.
The guy in the link you posted used unreasonable force. Quote what the judge said. Judge Barker told Hastings he had repeatedly stabbed Mr Williams in the back, the majority of the attack taking place outside the house when the possibility of danger was over.
PS You are allowed to use reasonable force in the uk including lethal force to protect yourself or family. Just like most countries that is fact.
Government advice quote. What if the intruder dies?
If you have acted in reasonable self-defence, as described above, and the intruder dies you will still have acted lawfully. Indeed, there are several such cases where the householder has not been prosecuted. However, if, for example:
having knocked someone unconscious, you then decided to further hurt or kill them to punish them; or
you knew of an intended intruder and set a trap to hurt or to kill them rather than involve the police,
you would be acting with very excessive and gratuitous force and could be prosecuted.