If you were at home a with wife and children in bed and an armed burglar broke into your house, you may well not be thinking straight enough during a struggle to decide what is proportionate force. Your only thought would probably be saving your own life and that of your family, even if it involved terminating the threat permanently.
The burglar had a choice in this situation, you didn't.
Allowances are very clearly made within UK law for the fact that it is an adrenalin-charged situation in which you may not be thinking entirely rationally. As they absolutely should be.
However, there is also an expectation that people should at least be attempting to avoid violence. This means that once the intruder is no longer a threat, you stop, because it's no longer necessary to be violent. So stabbing someone who is running away, or already on the floor with a stab wound and not getting up any time soon, is a big no-no.
This seems fair and pretty well-balanced, to me.