What sort of fare have you bought? What are the rules? Is it a changeable ticket or not?
BA fare rules don't allow standing by for an earlier flight on a cheap non-changeable ticket. If you ask, sometimes the answer will be yes, sometimes it'll be no. If the answer is yes, it will only be as a concession or goodwill gesture.
Of course, the better your reason for asking, the better your chances - my last success was a day of thunderstorm-related delays, when the immediately previous EDI-LHR flight was actually leaving at about the time that my booked flight was supposed to depart.