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).
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).
In R v Rashford  EWCA Crim 3377 it was held: The mere fact that a defendant went somewhere to exact revenge from the victim did not of itself rule out the possibility that in any violence that ensued, self defence was necessarily unavailable as a defence. However, where the defendant initially sought the confrontation (R v Balogun  1 Archbold News 3) ... A man who is attacked or believes that he is about to be attacked may use such force as is both necessary and reasonable in order to defend himself. If that is what he does, then he acts lawfully. It follows that a man who starts the violence, the aggressor, cannot rely upon self-defence to render his actions lawful.
Of course during a fight a man will not only strike blows, but will defend himself by warding off blows from his opponent, but if he started the fight, if he volunteered for it, such actions are not lawful, they are unlawful acts of violence.
Use of Force against Those Committing Crime
Prosecutors should exercise particular care when assessing the reasonableness of the force used in those cases in which the alleged victim was, or believed by the accused to have been, at the material time, engaged in committing a crime. A witness to violent crime with a continuing threat of violence may well be justified in using extreme force to remove a threat of further violence.
In assessing whether it was necessary to use force, prosecutors should bear in mind the period of time in which the person had to decide whether to act against another who he/she thought to be committing an offence.
The circumstances of each case will need to be considered very carefully.
See Public Interest Use of Force against Those Committing Crime, below in this chapter In R v Martin (Anthony)  1 Cr. App. R. 27, the Court of Appeal held that whilst a court is entitled to take account of the physical characteristics of the defendant in deciding what force was reasonable, it was not appropriate, absent exceptional circumstances which would make the evidence especially probative, to take account of whether the defendant was suffering from some psychiatric condition.
The final consequences of a course may not be relevant to the issue whether the force used was reasonable. Although, the conduct of the suspect resulted in severe injuries to another or even death, this conduct may well have been reasonable in the circumstances. On the other hand, the infliction of very superficial or minor injuries may have been a product of simple good fortune rather than intention. Once force was deemed to be unreasonable, the final consequences would be relevant to the public interest considerations.