This logic would render all national security reporting illegal. The job of national security reporters is to convince people in government to divulge information they're legally barred from divulging.
To me, that seems very different then what I normally think of as investigative journalism.
In my opinion I'm not sure how much or how heavy a book should be thrown at someone trying to decode a hashed password, but that's not really what is at issue. Perhaps the issue is, "is this a crime in the UK".