Generally, BA won't let you standby as a matter of policy like AA does on domestic itineraries. They will, if your ticket is flexible, change your booking if there's space and within the terms of your booking class for the appropriate fees.

There's anecdotal evidence of BA staff moving you onto another flight if one is oversold and there's plenty of space on the other but usually there has to be a sound operational reason for the staff to facilitate the change. (In fact, I've successfully changed my flights on a non-flexible discount economy ticket, however that was for the shorthaul sector from AMS only, but that's because my flight was oversold by 50 and had already checked in full whereas the flight leaving AMS sooner was going with empty seats and I was at the airport early enough).

Think of it this way... you'll have more time to enjoy the CCR at T5