The A330s they will use have a better onboard product than the A340s.
By taking out BMI, BA has done a fantastic job of turning LHR into an O&D only airport for non-BA aligned airlines. Even some historically BA aligned airlines have been pushed out of the hub model and forced to focus only on O&D traffic. Contrary to popular opinion/wisdom, O&D traffic alone is not everything, and SA has been forced to focus more on producing yield at LHR rather than volumes. While switching to the A330-200 from the A340-600 is a big drop in economy class capacity, it is a significantly smaller drop in business class capacity. So not only do they reduce the CASM but also probably increase the yield considerably.