Just to toss in one or two thoughts relevant to the debate between MrBond and Aneurin.
I think one issue is popular misunderstanding of the current implications of being arrested.
Everyone knows that between the presumption of innocence and condemnation as guilty there are several steps: 1. being questioned, 2. being arrested, 3. being charged, 4. being tried, 5. being found guilty.
But if I understand things correctly, the 1984 Police and Criminal Evidence Act made it much easier for the police to arrest someone. They now need comparatively little evidence against someone to have the legal right to proceed from step 1 to step 2 and actually arrest him.
Thus, since 1984 the gap between 1 and 2 (questioning and arrest) has diminished. But this has not changed the requirements for the other three steps. It is not significantly easier to be charged, tried or found guilty. Hence the gap between steps 2 and 3 has in fact increased proportionately.
I think the public remains widely unaware of this and assume that when someone is arrested, the police feel sure that he is guilty and have thus made a major error if they subsequently have to release him without charge.
Probably many people think, as I do, that the change, however desirable from the point of view of facilitating the police's task of catching criminals, raises considerable human rights issues. However, the police do not make the laws, and they will naturally take full advantage of the facilities the law gives them.
Meanwhile, though I regret this part of the 1984 Act, I don't think it in fact makes it more likely for an innocent person to be found guilty. We have either to change the law back again or get used to the fact that there need not be any more opprobrium attached to being arrested than there is to being interrogated. But it will take a long time for the public to take that on board.
I trust someone will correct me if it turns out that I have got my facts wrong.