Alternative Binding Ingredient for Sugar in Flapjacks? (British style)
This might affect the texture too much, but if you're willing to experiment you could try some mashed banana.

I made these banana and oat cookies a while ago, which have the ratio of 1 banana to ~90g of oats. The recipe says to blend the oats, though I didn't bother, and the cookies came out pretty chewy.

The recipe is way too heavy on banana to come out like a flapjack, but you might be able to use a little to offset a reduction in sugar in a proper flapjack recipe. I'm not speaking from experience though, so no promises that it won't be a disaster!