Some of the marijuana plants used in the shooting were real. The British government gave permission to the filming crew the use of 150 marijuana plants, under supervision of the authorities. This meant that the plants had someone guarding them on the set, and watching over its transportation to a nearby storage facility for its safekeeping at night.
When Dr. Bamford (Martin Clunes) and two townsmen witness the extreme phosphorescent lighting at Grace's (Brenda Blethyn) hothouse, the two townsmen want to call the local police and the RAF, for fear about Grace's well-being. Dr. Bamford discourages them, and advises then that Grace is helping apply her special growing techniques to certain medicinal plants. The townsmen ask Dr. Bamford whether he ever tried the plants, and he allows that he did, once, while in college, but he didn't inhale. That is an obvious allusion to former President Clinton, who famously indicated he tried marijuana during his Rhodes Scholar years in England, but didn't inhale.