Refusing to provide encryption keys is the same thing. There might be illegal data, there might not be. It's the duty of the police to prove it, not the accused.
Innocent until proven guilty? Not in the UK.
In the US courts are split on the issue. Some say giving an encryption key is testifying, an act of the mind, and you can't force someone to testify against themselves under the 5th. Others say its like handing over a regular key, which you can be forced to do, because the 5th covers testimony, not everything incriminating. It was intended to prevent forced confessions.
Once you're in front of a court you don't get to keep secrets, with the exception of some narrow protections. This has always been the case in the Anglo-American system.
You are not necessarily obligated to testify to that fact for the police, and unless they can demonstrate that the drive belonged to you (or you had access) through some other means, the production of an encryption key is tantamount to forcing that confession.
It's much like if there were a lock on a gun they found on the street: if they can't link the gun to you already, they can't demand you turn over the combination for the lock, because knowing such a combination would be a tacit admission to knowing about the gun.
from http://en.wikipedia.org/wiki/Right_to_silence_in_England_and... (and cite-note 15)
The case concerns a claim that it was injust to make inferences as to the guilt of the appellant based on their choice to remain silent before the police and court. This appeal under Art.6 ECHR failed (though a claim of preventing access to an attorney succeeded). The court finding that there was no undue inference made, that any inferences as to guilt that had arisen out of the defendants failure to break silence were allowable.
I'm not sure this really helps so much as it seems as it appears to allow the [partial] curtailment of presumption of innocence.
Of course we don't (completely) trust trust the judicial system. That's the point of half the Bill of Rights, for one.