I had noted that you referred specifically to Britain where glaciation could be used to to explain movement of stones over much it's area and assumed that you would be aware of the movement of much bigger stones over much greater distances than we are talking about here , e.g. Baalbeck , Collosi of Memnon etc . There is no argument about these efforts and glaciation can't be involved so we do have precedents for the movement and quarrying of stones but maybe we shouldn’t expect too much from the occupants of our little island .However the point is the importation not the size or effort .
Bringing something 5K no matter how big particularly when the same quality can be found locally is an important pointer to our understanding . Nevertheless some of the biggest stones at the Bru came from over 5km away .That haversack of quartz probably came 80 Km , that is what matters .
The recumbent at Old Keig could be an erratic but if it is, it had been quarried before glaciation and would therefore be a unique example of Paleolithic masonry .
Obviously builders of monuments use local material , but not all do .

I think there IS some evidence he was wrong. If you look at the sarsen drifts back towards Avebury you'll see they are all shapes and sizes and ones that are approximately the right size or shape for Stonehenge uprights (or lintels) are mighty rare. It seems a bit of a rum idea that only the right shaped and right sized sarsens existed near Stonehenge. There would be a vastly larger number of unsuitable ones as well wouldn't there? Statistically (and by reference to other drifts) the vast majority would be of no use - too rounded or hopelessly wrong-sized to be used as either uprights or lintels. Where are those rejects?

I have just had a quick skim through the Thorpe & Williams Thorpe paper and it is apparent that they merely done a quick survey not dissimilar to what archaeologists had noted since the early 20th C. i.e. that megalithic monuments are confined to areas where there are convenient local outcrops or rocky areas in general . They have not done a survey of each stone circle and the lithogeochemistry of each component ,an immense task and probably unnecessary as the basic assumption of the earlier archaeos is probably broadly correct that the builders used local materials exactly as you would expect . However there are some monuments where there is obviously not the case but as all these cases are in areas that have seen some ice cover or extreme glaciation they take the view that this explains all the anomalies . They are by no means as certain as you though in that they say “ most stone circles in Britain appear to have been constructed from local materials “ and “within the glaciated areas of N Europe it is more difficult to interpret because of the possibility of glacial transport “( my italics ) .They do mention Old Keig saying “the Grampian area has experienced repeated extensive glaciations ; the case for human transport , rather than selection from locally deposited glacial till is unproven “ I wonder , if by placing Old Keig within the Grampian area they are aware of the huge differential in glaciation in that political zone which contains the Cairngorms ,which were heavily glaciated but further north had an entirely different experience ( see “The Glacial Period in Aberdeenshire and the Southern Border of the Moray Firth “ Thomas Jamieson .) resulting in the previously mentioned lack of erratics . I doubt that you will find a local geologist claiming the recumbent to be an erratic. One sacred cow may have been taken towards to the slaughter house but it may be replaced by another , if any reasonable suggestion is met by “it was probably an erratic “ it leads to a conclusion that the inhabitants of previously glaciated northern Europe were somehow causally connected to these events resulting in an inability to move megaliths long distances whilst their neighbours to the south , possibly not lacking in the mineral deficiencies