Fortunately its nowhere near that bad. The prosecution needs to prove beyond reasonable doubt that the file contains encrypted data and that you have the key. So if you did have a file of random noise then the police would not be able to prove these things. And if you had software for using that file as a random number stream then that would be evidence in your defence. Remember, its for the police to prove your guilt, not you to prove your innocence. And the law in question does actually talk about that.

If someone wrote random noise to your hard drive then they couldn't prove these things (and in that scenario it would be a lot easier to frame someone by simply writing some child porn).

Encrypted files are not usually just cyphertext: there tends to be a header file with the file type, and of course there will be associated decryption software on the disk. Time and date stamps may well coincide (e.g. the decryption program was last run at the same time the suspect file was accessed). All of this would help build a case that the suspect had in fact decrypted the data in the past, and therefore could do so again.

As for steganography, they would need some clear evidence that the alleged steganographic data existed. Merely saying "maybe" and a pound will get them a cup of tea.