Oh come on. Punching someone in the face is also a crime but I can happily punch a rapist committing the act. The law is not black and white and this sort of thinking only damages discourse.
I can't speak for the UK, but in the US punching someone that is committing an illegal act, however noble your intentions to interrupt or stop the act, doesn't give you immunity to any civil or criminal charges raised by whomever you assaulted. You might have a pretty good defense for your actions, but you're not immune and you're not on the "right" side of the law.
> A person is justified in engaging in conduct otherwise prohibited if he has legal authority to do so.
In obvious cases, the person may not be charged, or a judge may dismiss the charges at the outset, but some cases of legitimate self-defense will inevitably go to trial for a jury to determine whether the person acted in self-defense or not.
For a start, even if this was the case, the CPS could not possibly bring forward a prosecution in the public interest. It's absurd on its face. Black and white thinking is usually fallacious and especially so here.
For instance, until recently it was illegal to make an audio recording of another person without his or her express permission. These laws were put in place to prevent criminal "wiretaps" (for purposes of blackmail and the like). Of course every parent who recorded his daughter's dance recital was also committing a felony, but since no one was ever charged for that, nobody complained.
But then the police realized that if they wanted to prevent someone from recording their illegal or embarrassing activities they could arrest the person for illegally wiretapping them. This led to a string of arrests and prosecutions for doing nothing more than documenting police brutality, incompetence or corruption. The point here is that laws have a way of being misused and some laws are easier to misuse than others. Censorship laws are in this category, so special consideration should always be given to the possible unintended consequences when such laws are considered.
Perhaps over-zealous or malicious prosecution doesn't happen in the UK, this seems a bit far-fetched to me, but maybe it's true. It certainly happens in the US.
Of course, if 1 in 6 men were victims of an attempted or completed rape, and if 9 out of 10 victims of rape were men,[0] I'm sure the conversations here would be very different.
[0]: http://www.rainn.org/get-information/statistics/sexual-assau...
So take statistics like that with a huge grain of salt.