Flex Plus fares have no change fee and no cancellation penalty - it is not specified that these book into a specific inventory

From BKK-LHR there are fares that have no penalties that book in A class .

When making a purchase the site automatically gives the lowest fare for that fare type. If A class is unavailable for the desired new flight, then need to pay the fare difference to F class.

You have not lost any flexibility - just have to pay the F fare . Most people , I suggest, would not have wanted to have been charged the extra for an F fare rather than pay the A fare and then the fare difference should it be needed