Right you are. That's why I suggested that the OP might go LAX-LHR on AA rather than BA, then transfer to BA for LHR-PVG or route via Canada. Of course, the OP ought not do that since LAX-LHR-PVG isn't really a practical itinerary in terms of time (must overnight at LHR on the return) or price (very pricy) compared to the usual Pacific routing.

The basic answer to the OP's question is that the only OW airline with economy plus is BA and they don't fly that route.

Michael