alaskaair.com cannot show BA flights, just AA and DL. You will need to call the Partner Desk for those. BA flights will charge you whopping huge fuel surcharges (something close to the cost of a coach ticket when you add in all the taxes for a premium ticket).

The reason why you don't see DL inventory is because DL doesn't offer any inventory*.

* this is only a slight exaggeration. Go over to the DL forum for discussion on why DL wants to charge you 300,000 SkyPesos™ or more to fly to LHR in J. You might try looking at BOS-LHR, though.