Just to answer my own questions:
I called BA, and the Expedia locator was indeed different from BA's. After getting my "real" locator number, I was able to look at the seat maps with checkmytrip.com and saw that row 20 is definitely in WT+ on the SFO-LHR 744 config.

[This message has been edited by DaveSF (edited 11-30-2002).]