I have never ever heard a common British person call it marijuana or cannabis unless it was in a sarcastic manner as in "Oop you smoking a bit of the marijuana?"
It's always weed, lean, green or something other slang term.
Well, this was in the context of a narrator clinically (yet also overdramatically) describing a person developing a "serious habit" of smoking weed, as if it's the same thing as being addicted to fucking heroin or crack.