Just checked my seating again.
Bit strange - when I reserved the seats I was in 39C which was the twin at the back.
The flight to BNA is now the 789 and so 39C was in the middle of economy! Glad I checked. I managed to change to the front window exit seat at 30A.
39C remains the twin row on the return flight. I am considering the upgrade outgoing. I’m also wondering if there is a deal to be had at Heathrow on the upgrade better than the online offer even if that at £239 isn’t much already.