The Secret Intelligence Service (SIS), commonly known as MI6 (Military Intelligence, Section 6), is the British intelligence agency which supplies the British government with foreign intelligence.
The organization makes an appearance in Hellblazer: Pandemonium where they frame John Constantine for the bombing of the British Museum by terrorists. They forced John Constantine to help them combat a djinn that's been wreaking havoc for the US Army in Iraq. MI6 is portrayed in the comic as a brutally efficient agency who's not shy away from violating the human rights of their prisoners (they beat John Constantine up and even threatened to ship him to Uzbekistan to have testicular surgery), John agrees to help them together with a Middle-Eastern spy named Assera Al-Aswari who works for the MI6. The magus succeeded in trapping the djinn, but as he returned home to England, he then used it to force the MI6 to leave Assera Al-Aswari and him alone.
In Constantine: The Hellblazer issue 3, an British secret agent and former confidant of John Constantine named Georgiana Snow, helps him fight a supernatural monster that's been targetting John's ghosts.