It is possible on BA issued tickets but typically is not offered, you can ask, having EI status or a tight connection at LHR normally result in being moved upon request. Its much easier coming back LHR-DUB where unoffical policy is to get you back to DUB as quickly as possible if on a J ticket or status holder

I just did this in LHR this morning, no issue at all at check in desk, if you hold status or plus/advantage/aerspace its down to seats being available, i'm 2 flights earlier than booked as a result but that was an EI issued ticket