The cigarettes smoked in the film (particularly by Tommy) were specially made-for-filming fake cigarettes. At the TIFF screening, director Pete Travis joked that Riz Ahmed was sick whenever he had to smoke them, in which case Travis would then make him smoke another one for good measure.
The actors playing the younger versions of the main characters had almost no acting experience between them. One of them was enrolling into drama training when she was offered the part.
According to director Pete Travis, Riz Ahmed was his only choice for the lead role, and wouldn't have done the film if Ahmed had refused. Luckily, Ahmed accepted the role, and by the time the film went into production, his clout as an actor had increased dramatically.
Room 512 (where Tommy is told Natasha near the start of the film) is perhaps a reference to The Wonderstuff's song of the same name.