That's an oxymoron: BA Avios redemptions on AA always book under the AA prime flight number, never as a BA codeshare. I can find AVL-XXX-LHR award space for Avios redemption in Economy, with the AVL-XXX leg on AA, and the XXX-LHR leg on BA. I can also find purely domestic redemptions out of AVL on AA using BA Avios. But as noted, it will be a real challenge to find AA TATL partner-award space in Business.

Is there any reason why you want to fly AA rather than BA for the TATL leg? When redeeming Avios for Business, the cash surcharges between the U.S. and London will be hefty regardless of whether you fly AA or BA on the TATL leg.

Edited to add: And, of course, you won't find any AA flights available today for travel next October, regardless of whether you want to pay cash, Avios, or AAdvantage miles. AA starts selling seats on its flights 331 days before departure.