Southwest does two non-stops a day each way between Balltimore and San Diego and at least one one-stop, no change of plane.
http://www.southwest.com/
http://www.southwest.com/cgi-bin/buildItinerary2?hps=b1
BA does LHR to Baltimore IIRC.
No change of airport is needed at Baltimore, though international arrivals and Southwest are at opposite ends of the terminal. Baltimore is a major site for Southwest with about 164 departures per day. WN logo plane (half of Maryland state flag)
http://www.southwest.com/about_swa/p...yland_one.html