Can you not do F out and J back?
You'll find a way to get the Avios for next year.
F on an over nighter is a waste IMHO. I've even turned down a J to F upgrade as I prefer the 747UD (64 A/K).
Enjoy the CCR and the 'door' going. Coming home, a J bed is fine. F is not much better once flat. F is, after all, all about the service.