Catching up, these reports are nearly two weeks old. Basically:
Aimen Dean is a founder member of al-Qaeda, who changed tack in 1998 and became a spy for Britain's security and intelligence services, MI5 and MI6.
There is a partial transcript of a BBC Q&A radio interview, by Peter Marshall (one of the BBC's best journalists), which touches on many issues:

Note how his role ended:
Valued first by al-Qaeda and then British security and intelligence, Aimen Dean's life under cover came to an abrupt end when the cover was blown. An American writer disclosed his identity with details that could only be sourced to Dean. That was eight years ago.
It appears he was a spy from 1998 to 2007. Perhaps a book will appear next?

A short non-BBC report:

Aimen Dean was interviewed on BBC World's Hardtalk, which asks:
What does his extraordinary story tell us about the nature of the jihadist threat?