The best "defence" against the glacier theory (assuming we all have a dodgy non-scientific wish for the "human transportation" story to be the right one, which I do) is to desist from asking "why would the Stonehenge People go all the way to Preseli to get stone?" and to be much more positive about the evidence and to say, confidently - "it looks very much as if the Stonehenge People went BACK to Preseli to get their stone circle".