Our criminal justice system achieves a very high conviction rate. Magistrates courts deal with around 1.75 million defendants a year. Only 10% of these are contested. The prosecutors have got better at weeding out the cases for which they are unlikely to secure a conviction. The overall conviction rate is so high (c.99%) that the Ministry of Justice does not publish it in the court statistics. In the Crown Courts, where the more serious cases are heard, over two-thirds of defendants plead guilty. However, of those that plead not guilty, 61% are acquitted.

Those who want to be tougher on crime can point to the increasing number of convictions. The police would say that they have no interest in taking an innocent person to court. They act on the presumption of guilt. Those who favour the importance of human rights can point to the high acquittal rate. They are activated by the presumption of innocence. The presumption of guilt is compatible with the presumption of innocence because they are held by different institutions: prosecutors and courts respectively. The presumption of innocence is a legal device whereby the burden of proof is placed on the prosecution.
The DNA database is said to undermine our civil liberties. However, it is not apparent that it strengthens the presumption of guilt. There will be more convictions since more perpetrators of crime may be detected. There will also be more acquittals since DNA is capable of absolving the accused as well as providing compelling evidence for the prosecution.
More pressingly, the presumption of innocence can be eroded prior to the trial if the evidence is gathered unfairly, thus undermining the requirement for a fair trial. Investigators can take short cuts or break the rules. The rules of evidence are notoriously complex in common law countries like the UK and USA. Defendants should only be left to represent themselves in the most straightforward and minor of cases. Where liberty or livelihood is as stake, then a professional defence is essential. For those who are unable to pay for their defence, which is most people before the courts, adequately funded legal aid is essential.
The problem is that those who set the budget for legal aid are influenced by those who see the criminal justice system as a kind of conveyor belt driven by performance targets. It may only be when we get too many miscarriages of justice that there will be any political change of heart.