I suggest that you check-in online at T-24 hours for your first flight but don't print / download your boarding pass. That way you can switch seats if a better seat becomes available. As you've booked a BA holiday you won't be offered an upgrade when you check-in on-line but suggest that you ask at the airport about prices to upgrade (although ask them to check about availability of seats together if you do upgrade if that is important to you.
I wouldn't pay for an upgraded meal but instead would put that money towards buying some of my own snacks for the flight (and / or some BOB on the connecting flight). You also might want to explore paid lounge options at EDI or LHR if you'll be there for long enough to justify it.