It is interesting to note that trial by jury, the right to a fair trial and the right to not be detained without charge have all been part of the British legal system since about 1215.  This was the time of King John and the Magna Carta.  Prior to widespread use of trial by jury suspects experienced trial by ordeal, which is similar to the witch trials in the middle ages, in which suspects were dunked in a river.  In trial by ordeal, God judged the outcome of the physical ordeal that was inflicted on the suspect, however other more tangible psychological events took place (eg confessions).  If the suspect survived the ordeal then presumably God had made his judgment, or as the wikipedia article suggests, if a person was prepared to face the trial, then the trial was often rigged so that the suspect survived.

Trial by jury is a good system because it means that justice is not vested in any particular socio-economic group, which would be the case if all cases were decided by the judiciary.  In the Bain case, this is why it is so important that there was a retrial in front of a jury and why the Privy Council ruled that the Appeal Court was out of place in turning down the many appeals.  It is why they granted a retrial.