Pre-emptive strikes
There is no rule in law to say that a person must wait to be struck first before they may defend themselves, (see R v Deana, 2 Cr App R 75).
Retreating
Failure to retreat when attacked and when it is possible and safe to do so, is not conclusive evidence that a person was not acting in self defence. It is simply a factor to be taken into account rather than as giving rise to a duty to retreat when deciding whether the degree of force was reasonable in the circumstances (section 76(6) Criminal Justice and Immigration Act 2008). It is not necessary that the defendant demonstrates by walking away that he does not want to engage in physical violence: (R v Bird 81 Cr App R 110).
An offence of Common Assault is committed when a person either assaults another person or commits a battery.
An assault is committed when a person intentionally or recklessly causes another to apprehend the immediate infliction of unlawful force.
A battery is committed when a person intentionally and recklessly applies unlawful force to another.