With a downgrade you can ask to be moved to another BA service, say tomorrow, but normally they won't allow a rerouting as well as the use of another airline. A cancellation would be different, so if you are seeing a cancellation email (as opposed to some sort of downgrade notification) then other options would be possible. But they will allow rebooking onto AA + BA services for connection purposes, so for example going via PHX or LAX which do have CW seats to London. You are only allowed one bite of the cherry here, once you've accepted a particular itinerary then you can't (again normally) change it if something better turns up. It is also possible that CW seats will show up at the last moment on your original service.