The article doesn’t say, but I wonder if any work has been done to try to chart the direction of British glaciers? The Preseli Hills are West North West of Salisbury. The glacier would have to have been basically traveling East to move the stones to Salisbury. But I have always been under the impression the glaciers were a North-South thing.
Yes, that's a great big hole in the hypothesis, apparently.
At a global level of analysis that's true.
At a smaller scale, more local topographical features may be the major determinant of the direction of glacial advances/retreats.