Flew MEL-LHR and back last week in J, and in both directions they had priority disembarkation on the SIN-LHR leg, but none on the MEL-SIN leg. In fact it was pretty annoying on the SIN-MEL leg, as c-class cabin was over-run soon after landing, making it a struggle to get baggage out of over-head bins.
Perhaps they are only doing it on the 747 routes ? (the MEL-SIN legs were on a 777-300)