There's no such thing as a standard gate for the majority of flights. If you want to be at the gate when boarding commences (rather than at the end) - then aim to be at the gate at around T-30. It'll have been announced well before that. Allow 10 minutes to get to a B gate (assuming a fair walking pace) or 15 for a C. If you take the walking tunnel - it might take a little longer. I normally do this and find myself standing at the gate for 5 minutes before boarding starts.

In terms of chosing a meal - it depends what you're doing. If it's a special meal - then yes - that rules out op-ups. If it's the pre-booked meal that BA now allow - then no - that doesn't prevent op-ups (I can testify to that) - but equally - it won't carry forward into the new cabin for you, so you might have picked the chicken, it might still be on the menu, but you might find it unavailable.

However, as a couple, your chances with the upgrade gods are much lower anyway. BUT the 219 on the 12th is busy at the back:

F7 A7 J9 C2 D2 RC IC W6 EC TC Y4 B2 H0 K0 M0 LC VC SC NC QC OC G0

Although WTP does look like it has space free - but you never know.