If you go to the Advanced Search Page you can select American Airlines in the Options section at the bottom of the page. That will prevent BA flights being returned. Obviously I don't know how many seats or what cabin you want but right now there's a couple of days in early to mid May with >1 Business MileSAAver on AA metal (5/6 and 5/14) from WAS-LHR and a couple of dates mid to late May (5/14, 5/15, 5/21, 5/22) with the same thing on the return.