I am flying DFW(AA)-LHR(AY)-HEL(AY)-SIN and have a 6 hour layover in LHR before catching my flight to HEL. There is an earlier flight from LHR-HEL, would it be possible for me to fly standby on the earlier flight.

I will probably have to get my boarding passes for my LHR-HEL-SIN flights from the transfer desk at T3 at LHR. just wasn't sure if anyone has experience on getting standby on a earlier flight while flying international.