What's the alternative to a jury? A case hardened judge? Juries do not always get verdicts spot on (though they cannot be challenged), but I think everyone would agree they are preferential to a judge who has heard every excuse in the book and will never listen to them - try pleading not guilty in a mags court and see what happens. I have heard arguments for professional jurors, but we simply do not have the resources to invest in training and maintaining them. In any event, they would just become case hardened in time anyway.
Jurors often do not understand the law in more complex cases such as fraud, but for relatively easy to understand cases, such as ABH, they can get their heads around it and decide on a verdict easily - particularly when they have the expert direction of an experienced judge and arguments from barristers to consider. If you feel justice has not been done because of a lack of evidence/unfair sentence etc, go back to your solicitor/barrister and think about appealing. You can appeal either the conviction or just the sentence, but be advised that if the evidence is overwhelming then the conviction may be upheld and your sentence could be increased.
Hope that helps.