I would wait and see what happens on the day, I have often got onto earlier flights at the airport, the main issue is checked in luggage though, you need to make sure it only gets tagged to LHR.
I managed to switch to an earlier flight the other week, but only by calling the euro BAEC no, they could not do it in the uk, guess it was one on those depends who answers the phone issues again.
I don't think you will get anything back on the domestic leg, but I would wait to the day and see. Good luck and have a great flight.