From another UK source
What are your rights if someone breaks into your home?
The CPS and police urge people to always call the police first - if you are able.
But according to official guidance, anyone can use reasonable force to protect themselves.
As a general rule, the more extreme the circumstances and the fear felt, the more force you can lawfully use in self-defence.
The law doesn't require you to wait to be attacked before using defensive force, but does not protect you if, for example, your action is over-the-top or calculated revenge.
You are given greater protection under the law if force is used to protect yourself or others when dealing with a burglar or trespasser on your property.
The CPS says if you have acted in reasonable self-defence, as described above, and the intruder dies you will still have acted lawfully.