There's sometimes a game of chicken here, particularly on routes like LCY-EDI where Theoretical Seating isn't likely to work since there could easily be half a dozen GGLs and Prems on board, never mind common-or-garden Golds. So both 2C and 2D gets selected, CE isn't full so it's the only pair of seats occupied, and then someone in that pair bails out at the last moment or on board, taking an otherwise empty pair of of seats further back.

There is indeed no right answer, usually there will be at least the choice of the row 2D/F legroom or an empty pair in CE. Luckily the rest of the 190 seating is sufficiently spacious that no one really loses out. The obvious point is that one can stick to 2F and then an hour or so before departure select something else if you haven't downloaded your boarding pass. If it's definitely not going to be a full flight then you can use TS from 72 hours onwards to make an educated guess as to whether your TS block will last through to boarding or not.