Question: Is there some reason that we are considently seeing planes fly over her house? Is that supposed to symbolize something?
Answer: I doubt it; it's probably because the film is set in the Borough of Hounslow, which is in the flight paths of many London airports, including Heathrow.
Question: What happened to Jimmy's brother in the movie?
Answer: It's never fully revealed, it seems David and his brother were playing, probably doing something that requires parental supervisor and was unable to save him. Again, probably David was the older one and his father believed he should have known better.
Question: What's the point of the orange and the marijuana? I didn't quite get that.
Answer: I've been told it's to keep the marijuana moist. The moisture from the orange peel keeps the stuff fresh so that it burns longer. The same happens with an apple peel.
Answer: Jess' house is supposed to be on the main flight path of Heathrow airport. Her mum and dad work there.