Bob committed an offence if Bob hit Andy just because Andy dared him to.
Bob didn't commit an offence if Bob hit Andy for the purpose of self-defence, with the honest belief that the force he used was necessary and reasonable in the circumstances, and if a reasonable person wouldn't regard the force as excessive. (I assume Bob will persuade police/prosecutors/court about these elements.)
The only defence of provocation was to murder. This defence was abolished when s56 of the Coroners and Justice Act 2009 came into force in October 2010. It was replaced by the more restrictive 'loss of control' defence in s54. Neither provocation nor loss of control are a full defence, they are only mitigatory or 'partial': if successful, the person liable to be convicted of murder would instead be liable to be convicted of manslaughter.
Many if not all use of force questions about hypothetical circumstances can be answered in the same way.
Quoting the Crown Prosecution Guidance on Self-Defence and the Prevention of Crime, reasonable force:
A person may use such force as is reasonable in the circumstances for
the purposes of (in the alternative): -
- defence of another;
- defence of property;
- prevention of crime; or
- lawful arrest.
In assessing the reasonableness of the force used, prosecutors should
ask two questions:
- was the use of force necessary in the circumstances, i.e. Was there a need for any force at all?; and
- was the force used reasonable in the circumstances?
The courts have indicated that both questions are to be answered on
the basis of the facts as the accused honestly believed them to be (R
v Williams (G) 78 Cr App R 276), (R. v Oatbridge, 94 Cr App R 367).
To that extent it is a subjective test. There is, however, an
objective element to the test. The jury must then go on to ask
themselves whether, on the basis of the facts as the accused believed
them to be, a reasonable person would regard the force used as
reasonable or excessive.
It is important to bear in mind when assessing whether the force used was reasonable the words of Lord Morris in (Palmer v R 1971 AC 814);
"If there has been an attack so that self defence is reasonably necessary, it will be recognised that a person defending himself cannot weigh to a nicety the exact measure of his defensive action. If the jury thought that that in a moment of unexpected anguish a person attacked had only done what he honestly and instinctively thought necessary, that would be the most potent evidence that only reasonable defensive action had been taken ..."