[edit] actually reading the actual law, the wording is being "in possession of a key", and the police has to prove that you are in possession beyond any reasonable doubt. Can a word in someone's brain be characterised as being in possession? And how can the police prove that someone remembers something?
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.