Anyone can use reasonable force to protect themselves or others, or to carry out an arrest or to prevent crime. You are not expected to make fine judgements over the level of force you use in the heat of the moment. So long as you only do what you honestly and instinctively believe is necessary in the heat of the moment, that would be the strongest evidence of you acting lawfully and in selfdefence. This is still the case if you use something to hand as a weapon.
As a general rule, the more extreme the circumstances and the fear felt, the more force you can lawfully use in self-defence.
Do I have to wait to be attacked?
No, not if you are in your own home and in fear for yourself or others. In those circumstances the law does not require you to wait to be attacked before using defensive force yourself.
http://www.cps.gov.uk...ion/householders.html