Thanks, mzb00. I will wait (I was expecting the flurry of responses I get on the BA or AA forums). My assistant called and they said the plane on that route is now fully flat. I'm skeptical but will probably take it and take the risk. I'll be flying LHR-GRU on the way there but in F on BA, so certainly lay flat.
I took Avianca from LHR-BOS-MEX in January (?) and that was flat, I'm pretty sure.