In other words, the defending party may do whatever it takes so long as no alternative, less severe options are available (except fleeing the immediate area NJA 1969 p425, 1999 p460).
The expression "blatantly unjustifiable" allows fairly generous tolerance towards the defending party.
Loss of life or permanent bodily injury is rarely justified as self-defence unless the defending party was in danger of being subjected to the same.
This is because that which is defended (property) is not as valuable as a human life and therefore the action of killing the criminal is not justifiable.
For instance, such a situation might be if the defending party were in a state of great fear or severe rage because of the peril.