You're guilty until you can prove otherwise. How can you prove without shadow of doubt that you don't have the keys in your brain?
My point is from the other direction. Wouldn't they have to show some proof or evidence that you have anything to begin with?
Well if we take a look at the law in question:
For the purposes of this section a person shall be taken to have shown that he was not in possession of a key to protected information at a particular time if
(a)sufficient evidence of that fact is adduced to raise an issue with respect to it; and
(b)the contrary is not proved beyond a reasonable doubt.Not being a lawyer, it sounds to me like the burden of proof is on you.
Otherwise they'd have to go start arresting anyone and everyone on suspicion that they might have something in their head.
Well that is probably the idea. The government loves it that way, total power over you.