If you end up looking at Y fares, remember to try kulula.com. It often sells tix on the BA-branded aircraft as well as the kulula fleet itself. You're looking for 4-digit MN flight numbers that correspond to the digits of the flight numbers of the BA-coded flights, and the same arrival and departure times.

Reportedly, once you're on the aircraft you get the BA service.

These tend to be more expensive than other kulula flights, but cheaper than buying them on the BA code. Obviously, you don't get the other BA perks, but if cost ends up being important this is one way to get a "free upgrade".