Just to add, you don't have to know you are in immediate physical danger to defend yourself. You do not have to wait until you are attacked before you defend yourself. You cannot kill unnecessarily, but if you believe you are in danger, and that belief is deemed reasonable by the police, given the circumstances you were in, then it is not an offence to hit/stab the intruder.
It only becomes a crime if you are grossly disproportionate, such as striking them when they are unconscious, chasing them down the street if they run away and so on.
It would also be a detriment to your defence if you just "happened" to have a knife under your pillow as this would suggest you were prepared for an attack (I believe one of the main reasons why Tony Martin got convicted). But if you came across an intruder in your kitchen and grabbed a knife in a panic, that would be fine.
The key point to remember is that it's not the outcome that determines whether an offence is committed by the householder, it's the intent. Rushing an intruder and bundling them to the ground, killing them as they smack their head could well be seen as reasonable, firing a shotgun at them (but missing) as they were getting into their getaway vehicle would probably not be.