Presumably, people who book to fly to/from LGW do so because that is where they want to fly to/from??? I guess LCY was an option at the time of booking?
Doesn't seem to be unreasonable to expect the pax to wish to retain the same airport pairs? Why wouldn't BA offer flights on their codeshare partner's flights?
Is LCY-DUB still flown by Flightline???
Maybe there's more money in a 'codeshare' pax at LGW, than a 'wet lease' one at LCY?