Please pardon me if this post is in the incorrect forum.

I'm looking to use FF miles to fly Kauai to LHR and return, with stopover on return in LAX; departing next August 31 and returning to LAX on Sept. 27 but not returning to Kauai until Dec 15.
I could fly AA or United, and prefer business class. How can I ensure a business seat? The dilemma is that I can 't book the full ticket until 330 days from the return, and so that leaves many months for other flyers to book the outbound flight, and the popular lhr-lax return flight. Any suggestions are greatly appreciated! Thanks.

Daniel