Certainly in the UK, and I believe in the US (whose system derives from ours), a simple statement under oath is evidence, and has to be countered by stronger evidence.
So, if you were in court under Section 49, and said under oath "I do not possess the key", it would be up to the prosecution to demonstrate that your statement was not believable (for example, by showing evidence that you had decrypted the file recently).
It's one of the things that, until a recent discussion with a lawyer, confused me about the legal system here; "sufficient evidence" apparently just means "will swear under oath, and has convincing explanations that counter any evidence presented by the other side". So, the police claim "nybble41 has hidden encrypted terror instructions in his photographs of a cat"; you can literally say to that "no, I didn't", and you've presented sufficient evidence.
It gets more complex if the police have more than just a bald statement; for example, if the police said "we saw nybble41 run 'convert catphoto.jpg -cdl 42.txt catphoto.png' and we believe that he was inserting encrypted instructions from 42.txt into catphoto.png". You could then explain about ImageMagick color description lists, and still convince a judge you didn't have the key.
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