perhaps (offhand guess) the geologists envision a large mass of bluestone broken up at the source and distributed widely over a very large area, including the Salisbury Plain. However, then the next logical question might be where are all the other large bluestone liths that should in theory still be lying around. in any case not having the full article is difficult.
This glacier mechanism idea has been pushed primarily by just the one guy, with occasional help. It’s got to be good for publicity, but basically it makes no sense, and never has. The burden of proof is definitely on the glacier advocates.