I believe that the situation is a bit more nuanced. Unless it changed recently .

BA "forces" its J pax to eat preflight at JFK for the overnight flights. That is the reason why only BA J pax can access the buffet dinner area (not BA Y pax with BA saphire/emerald status) . J pax are NOT served dinner onboard, just some little snack. So the situation is vastly different for J CX pax who get served dinner onboard.

There is a F section in the gallery business lounge that is open to OW emerald and CX F pax, but it is a joke as section is unfriendly and wine/liquor just marginally better. BA F pax (not BAEC Emerald or other OW emerald) are allowed to use the BA First lounge, which is in a different part of the terminal. There you can get à la carte dining. BA F pax have the OPTION to eat preflight (average food and wine) or onboard (with great wines and good food) or both. I usuall take some nice first dish in the lounge and full dinner onboard!