I'm sure something that you're contemplating anyway but Plane Food for the grub? Though I'm not sure there's anywhere that sells wine to take out...

If I had the time/inclination/capital, I'd consider setting up a shop in T5 selling little bottles of wine and other drinks. As long as BA's IT is as shoddy as it is I'm sure it would be successful.

Otherwise I think you might have to rely on the onboard shop working on the day. IME that's hit and miss in terms of both functionality and stock. It's pretty poor.