I'd like the option to begin as a second lieutenant in the RFC, but still be a rookie [i.e. I don't want this option to accelerate my promotion to lieutenant or to require me lead a flight during the first few weeks at the front]. This would simulate the middle or upper class Brit getting a commission on signing up.

In essence, with this option selected, during your current "sergeant" phase your rank would appear as a second lieutenant. Then, when you would otherwise have been promoted from sergeant to second lieutenant, you simply remain a 2Lt. There would be a false "promotion" from 2Lt to 2Lt, after which things would be back to normal.