I always found it funny that David's parents didn't come to London.

I know it would have derailed the movie -- David would hardly have gone home with Alex when he was discharged if his parents had come to London -- but you would expect most people's parents, if their son's best friend had been murdered so gruesomely that he'd need a closed-casket funeral, and their son was in a coma from the same attack, to have raced to London on the first flight they could get, and be at his bedside when he awoke.