The big issue for me is "continuum". Prehistory doesn't cease because literate Romans took over a part of Britain and wrote something down which we regard as "history" The ever so temporary Roman occupation was an interval during the ongoing Iron Age. What exactly do we mean by "megaliths" - big stones? What is the consensus view of the purpose of standing stones - waymarkers, boundary posts, memorials? Didn't they evolve into Pictish slabs and Christian crosses? Aren't they megaliths also?
I believe that the cut off point is just too early and much of interest is lost and so slips in through the back door eg Romano-British settlements often called "Celtic" villages, medieval mottes justified because there might be an earlier barrow inside, these odd currick cairns of which I know nothing but what I have read on TMA - are they not medieval or modern? Contributors are clearly pushing against the restraints and something has got to give sooner or later.