LHR has had the tray return system for years. Seriously, I flew from there in December 2009 and saw it then. They weigh the bin before returning it, and apparently it's pretty sensitive. As I was going through, I saw one try was stuck because a couple coins were stuck, and that's how I became 12 pence (3 small coins) richer. Still have them somewhere, seeing as how I wasn't been to LHR since!