Correct me if I'm wrong. There are very few cases when parallel rwy operations help in increase of capacity. It mainly has do do with ground handling capacity (taxiways, stands etc.)
I have been told that one rwy for departures and one for arrivals is the optimum configuration because it does not MIX different kind of flights (deps & arrs). Its this "mixing" which reduces capacity.
One for deps. and one for arrs. keeps a separated and organized flow of air and ground traffic.
Also I think Heathrow does not meet the 1500m parallel rwy distance requirement that is needed for such operations.