Scenario
The arrival gates for the north and south services are adjacent to each other.
The LHR CC bring the southbound aircraft in.The northbound aircraft arrived an hour before and has been cleaned and dressed.The southbound CC disembark their passengers,walk next door to the northbound aircraft(do the mandatory safety and equipment checks) and begin boarding passengers for a continuation of the northbound service to LHR.It is feasible and if found to be cost effective it will be introduced.Airline mangement are not benevolent people.All they care about is the bottom line