There are marked differences between the legal system here and in the US. Juries are restricted in the verdicts that they can return, not necessarily allowing cases to be thrown out because of sympathy for the defendant.
Peter would have been well aware that assaulting Donald could have dire consequence.
The police would doubtless press charges but the main consideration in my mind is that Donald has a lot of money and could probably crucify Peter for common assault, the outcome of which could quite possible cause Peter to lose his job, his livelihood, his house and then probably everything else along with it. A vindictive bastard like Donald would probably do so out of sheer spite. As it turns out Donald does seek revenge and it is left to Jenny to finally finish him off, something that will happen in the next chapter.
The law in the UK is an absolute ass, they let criminals get away with lenient sentences whilst persecuting misdemeanour with a vengeance. I'll give you an example
In a recent drug trial, a woman juror was found to have corresponded with one of the accused, a woman that she empathised with. She looked the woman up on Facebook and amongst other things, they discussed the ongoing trial. In the end, the woman defendant was acquitted but when the juror's activities were uncovered, she was prosecuted. Even though it was conceded that nothing they discussed prejudiced the trial, she was found 'in contempt of court' and was handed down a lengthy custodial sentence.
Please bear this in mind before castigating me over Peter not resorting to mindless violence.