I think that this is very difficult, because back-to-backs in this sense involve a second flight on the same aircraft as the first flight.

Because LCY is a base for BA CityFlyer, you cannot book a flight to LCY and be assured that the same aircraft will necessarily operate your next flight from LCY.

Over time, you can spot some patterns and some likely routings for individual aircraft, but these are far from guaranteed and can get changed at the first hint of operational trouble.