I was struck, when reading one of Aubrey Burl's books the other day, by the extraordinary lag that occurred between the use of big stones in megalithic monuments in Brittany, Scotland, Wales and Ireland on the one hand and in Wessex on the other hand. Big stones started to be used in all those former places around 6,000 years ago, but while a megalithic culture was flourishing on the Celtic fringe, the people who lived on and around Salisbury Plain carried on making timber monuments (were their trees bigger and better?) and long barrows made almost entirely with chalk rubble and soil, with a few stones chucked in if they happened to be handy (as at Boles Barrow). Often they are referred to as "earthen barrows" although some of them incorporate sections of stone walling -- generally using small stones. I have found some references to incorporated sarsens -- but very few.
I think I have this right -- no doubt I will be corrected if I don't -- but Burl mentions that of the 66 long barrows in the Stonehenge area only one (Tidcombe and Fosbury 1) has a fabricated stone chamber made out of sidewalls and capstone. West Kennet uses much bigger stones, and has a date of around 4,900 yrs BP. So the portal dolmens, passage tombs and variations were built over a wide geographical area in the Celtic Fringe, with only the Cotswold-Severn tombs impinging onto the chalklands of Salisbury Plain. This situation persisted for almost 1500 years, if we accept that the first stone settings at Stonehenge were not put in place until about 4500 yrs BP. Around about the same time, big stones were used at Avebury -- and after that, a megalithic culture then carried on alongside an earth-moving culture and a timber using culture. In the centuries around the Late Neolithic - early Bronze age transition, all three elements were incorporated into the big civil engineering projects of the Salisbury plain tribes or family groups.
Then, in the west and north, people got fed up with using big stones in tombs and moved into a "standing stone" phase instead, putting up rows, circles, ovals and pairs all over the place, not to mention thousands of single standing stones as waymarks, memorials, territorial boundary markers, cattle scratching stones, or whatever.
To summarise -- it seems to me that for about a thousand years, between 6,000 yrs BP and 5,000 yrs BP, big stones were used as a matter of course in burial chambers around the Celtic Fringe, but not on Salisbury Plain. Why? It's not that there weren't plenty of big stones lying around in the landscape -- David Field, Aubrey Burl and others have commented on the fact that there were sarsens littered across the chalklands. (I would argue that somewhere there were lots of erratics as well, but leave that to one side for the moment.....) People chose not to use them, but to continue with digging ditches, making ridges and embankments and putting vertical posts into the ground.
This does not argue for close cultural ties between the Salisbury Plain community and the communities of the Celtic Fringe. When, finally, a stone-based megalithic culture arrived on Salisbury Plain, it lasted for 500 - 600 years and was something of an aberration, with the creation of a rather wacky monument called Stonehenge, using woodworking techniques (tongue and groove joints, mortise and tenon joints etc) on stone -- and mimicking and developing the things people had been doing for many generations with big timber posts.
Very strange......and this does have a bearing on the likelihood of the Stonehenge people knowing anything at all about bluestones, the uplands of Preseli, and the routeways between West Wales and Salisbury Plain. There was clearly not complete cultural isolation, because stone axes and other trade goods were being exchanged all the time, but I would argue that that trading activity was more or less random, opportunistic and quite small in scale.