JER already served by SI.
I have no word on how SI are doing on this route, however it was brave of them to set it up in the face of stiff competition to LON, especially with the failure of VG on the route several years ago.
IMHO BA would make an absolute killing to LCY. An early JER-LCY, with an immediate return would pick up massive support from the many Frequent Fliers at either end of the route, who are still traveling to LGW with BA even tho SI already serve the route (points mean prizes!!).
Of course this would dilute the JER-LGW traffic, but in all honesty this route sticks out like a bit of a sore thumb and I think has potential to be moved back to LHR (it is, after all, mainly business traffic and a significant number of pax catching the bus to LHR)... But that's a different discussion for another thread.