Your will be on the LHR to GRU flight, i have taken that fly many times, the seats are angle flat beds but the are more than OK. Seats on the 777 are into a 2-3-2 config so if you are flying with someone you will be ok. You can look for some videos on Youtube for the 777, i have some but i have never edit them hehehehehe.

If you ask me i will choose to fly the Latam 777 biz before the awfull BA Biz seats, i dont like those seat because most of the time i fly with my wife and when i fly solo i dont like to be looking backwards

If you ask me i will try to take a fly with the GRU - Buenos Aires (Check for EZE and AEP) on a wide body, since on narrow bodys you have the European business class and on the fly that you put as sample the A321 dont even have that, only economy cabin.