The boulder discussed below is an excellent case where the terminology of found stones is important.A "natural" object is one that is found where nature has placed it and in its natural state.
A manuport is a natural object, especially a stone, that has been carried and deposited somewhere by humans but has not been artificially shaped.
An artefact is usually a simple object showing human workmanship or modification as distinguished from a natural object. It can still be in its natural place, for instance a rock carving.
When discussing glacial transport unless the natural place is known very little can be inferred from an object. Once it has been humanly moved it could come from anywhere.
The stone under discussion has marks on it and from the limited photos I have it is hard to tell what they are.
Some seem to follow the curve of the stone and appear to be natural foliations. Others appear to be more like incised lines. They might be scratches from glacial transport, or just striae of the rock structure.
If they are glacial then the rock is more likely to be a natural and brought to Salisbury Plain by ice; if they are just the surface of the rock then it is more likely it is a manuport brought from Wales by humans.
But if they are tooling marks where the stone has been dressed, and maybe it was knocked off a megalith, a little sibling to the "Boles Barrow" stone, then it is an artefact.
So skilled examination, rather than speculation from an imperfect photo, is needed to decide what this stone is. Only then can it inform us as to how it ended up under the ground at Stonehenge.