s.53(3):
"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."
In other words, if there's evidence for there to be 'an issue' about whether you actually do have a key (or whether e.g. it's just random noise), it's up to the prosecution to prove beyond reasonable doubt that it is actually data, and you do have the key.
So the flowchart is:
- If the police can prove they have reasonable grounds to believe that something is encrypted data that you have the key to, then
- That raises an evidential presumption that you do have it, which you can rebut by
- adducing evidence that just has to raise an issue about whether you have a key (inc. whether it's encrypted data at all), in which case the police have to
- Prove beyond reasonable doubt that it is encrypted, and you do have the key.
(IANAL)