One of the things that is not always dealt upon in this transport/glacial argument, if I may call it that;), is the fact that here beneath the Preseli mountains we have another well settled prehistoric area similar toStonehenge, with many ritualistic monuments; Glandy Cross comes to mind, plus of course many cromlechs, etc. The main ridgeway is seen as the overland route between Ireland and Wessex, and as such the area round this part of Pembs has a lot of prehistory, a probable trading route (bit like the Silk Road).
I could argue that people traded in stone, as they most obviously did in axes, bluestone axes have been found in the far west, so moving larger stones may have been a more ambitious project, but to be honest we don't know one way or the other.
I think Preseli has still to be analysed as to its prehistory, but Figgis's Field Guideto Prehistoric Preseli has plenty to be going on with...