When I lived in UK (many years ago) 'flapjack' was simply the 'American' word for 'pancake' in the same way that 'sidewalk' was American for 'pavement' and 'faucet' was American for tap. We knew these words because people said them in American films and TV shows, but nobody would ever have said that he or she was going to make 'flapjacks' for dinner.
We had 'pancakes'; they were large, flat ('flat as a pancake') soft, made in a frying-pan and could be tossed and eaten with (for example) sugar, lemon-juice or maple syrup. (Yes, we knew what that was). Everyone ate them on 'pancake day' (Shrove Tuesday, the day before Ash Wednesday) and we even had pancake races, where people ran over a course, tossing pancakes as they went.
No doubt the admen have been busy in the last few years and have introduced something called 'flapjacks' to the British spending public, though what they are is beyond me.
MK