Where do you mean by 'in the Trough of Bowland'? Let's call it Sykes Farm at the eastern end of the pass. In that case, the
Bowland Wild Boar Park site is 9km from there, or 6km south of Dunsop Bridge. I've not noticed any closer to Sykes when cycling through there, but I'm never looking for camp sites, so that doesn't mean there aren't any. Scorton, etc. are about 15 miles down the road, and the road goes over two sizeable hills so not 'close' as such really.
Marl Barn is a fair bit further than the Wild Boar place but has a pleasing, two dimensional owl sign :-)
It rather depends on where you're coming from and going to really.