In short you can sell anything if the flight was undertaken as a hobbyist with no anticipation that some random someone might want to buy them. Then you are free to sell them.
It is “the flight”, itself, that the CAA can consider as “commercial”, not the photos or video themselves. And they cannot retrospectively class a flight as commercial.
PfCO/Commercial, etc., are relevant only if the reason the flight was undertaken was with a view, an intention or an anticipation of receiving payment or reward, in any manner, as a result of that flight.
Since the start of the new year, has this changed? Given that the restrictions are MTOM based and not commercial vs recreational.
Could I now fly (within the correct parameters for my drone) and then speculatively offer the photos for sale? Or even offer photos and THEN fly with every intention of selling?
Pretty much yes, but the CAA take the view that if you’re flying a drone commercially you need different insurance to if you are flying recreationally or for sport. See CAP722 section 1.4.