Welcome to the BA forum, joshf. No, there is no formalised policy, nothing that you can rely on. There's also no standby or SDC facility on BA. However what you can do is book a LHR-NCE Plus fare (not Hand Baggage Only / Basic fare) for a good long connection, then so long as you are more than 1 hour from an earlier flight, and there are seats available, you can move free of charge to that earlier service so long as there are seats available. If you book it too tight then you are at BA's mercy, hence go long rather than short.