I agree. The surface stones will be like the tip of the iceberg -- I wouldn't mind betting that there are hundreds if not thousands more "bluestones" lurking in ancient deposits in valleys all over Wilts and Somerset. The surface ones around Stonehenge were collected up, because the builders had a job to do.
You may be right, but is there any evidence to support this, or indeed, evidence of any stone from south Wales being found in western England.