If you're flying UA from EZE to IAD, that would be a 9pm departure? At least it will be in July when I make the flight.
Even if you cut it close, you're dining in BA at 6pm - probably not a lot of good options at that time for a nice dinner?
I usually end up asking for a late check-out, take a shower before I head to the airport and, as a previous poster suggested, grab some empanadas.
You'll get fed as soon as you take off from EZE.