> faceless people at the British Establishment Country Club
That's roughly it.
The British Establishment (leaders of the main parties, intelligence officials, senior judges, senior police) wanted to make an example of him, because of his persistent exposure of their arbitary persecution of Assange. He was charged with contempt of court, for "jigsaw identification" of a witness in another case, that of Alex Salmond. That charge was prosecuted before a single judge, who was very much a member of "The Establishment", and he got 6 months in the slammer.
It's still not clear which remarks he made on his blog that contributed to jigsaw identification. Maybe it was a commenter on his blog that made the troublesome remark. But apparently other, more-mainstream journalists had been much more explicit, and most of the Scottish political class knew who the witness was from those sources, not from Craig's blog.
Basically, he was fitted-up for loudly espousing Scottish independence.