Presumption of innocence has been one of the foundation stones of fair justice since the Middle Ages in Britain, and has made us respected around the world for our constitutional liberty ever since.
However it is under attack, not completely by the law but from the surveillance state that has slowly been created in the past 11 years.
We are the most watched country in the UK, with one camera for about every 14 people. Do we really need that many cameras when, according to a chief constable, they only help with 3% of crimes?
The government has many databases, including the DNA database that holds your details whether convicted of a crime or not - so you could be locked up on pure suspicion of an offence that you would never be convicted of, but still lose your liberty. We have a database of phone calls coming soon, which is simply an infringement on personal liberty. It might catch petty offences or minor instances that might make the headline of the Sun (take an MP calling another woman regularly, for example) - but criminals are far craftier - and not to mention the ease at which they can be left on trains, or lost in the post.
The real point is that such surveillance, and I only include 3 examples, treat every normal citizen as a criminal. They have to be watched to make sure they do nothing wrong. That is a presumption of guilt, not a presumption of innocence. Believe the "nothing to hide, nothing to fear" brigade if you want to, but these people are simply naive.