The only reason BACityFlyer would operate MAN-LCY is to compete with VLM since they are now part of CityJet. I think you are all reading a bit too much into any possible link between MAN-LCY and LCY-JFK.
There are a lot of obstacles that need to be overcome before we ever see a BA aircraft operate to JFK from LCY and there are a significant number of BA staff that do not expect to see the route start anyway!
BA have never marketed LCY as a transfer hub (despite it being a good alternative) and I am sure they would rather see you go through LHR even LGW.
If you look at the recent BA CityFlyer expansion and any new routes are either already operated by CityJet or in one case they are indirectly competing with Air France itself.