Apologies in advance if you guys know about this person already or have discussed him elsewhere on this site (I couldn't find mention of it, if so), but a Belgian historian-cum-outsider-archaeologist named Marcel Mestdagh made some interesting megalithic discoveries in France that remind me very much of your own in Britain.
Mestdagh's work isn't translated into English from what I can tell, so this is all secondhand, via Philip Coppens, whose book The Lost Civilization Enigma I have just been reading. Coppens says Mestdagh became dissatisfied with the standard historical account of the Viking invasion of England and France, specifically thinking there was something odd about the path the Viking army took in its conquest. He finally realized that the Vikings had been following an ancient system of roads radiating from the towns of Nottingham, England, and Sens, France--roads he initially thought were Roman but later realized were originally megalithic pathways, because they were thick with standing stones (which he interpreted as boundary markers, but whatever...). The Vikings ended up taking Sens in an oddly peaceful siege in 886-7, and it seemed to Mestdagh as if the Vikings were searching for something--the legendary Valhalla, specifically--using megalithic knowledge that they had retained knowledge of, and that they actually found it (or what had become of it) at Sens. Like you, he argued that the Druids had simply retreated to the European periphery (i.e., Scandinavia) after Anglesey, and that the Viking conquest represented a kind of reassertion of Megalithia (I don't think he calls it that) after all that time, not just haphazard rape and plunder.
Most interestingly, Coppens says Mestdagh spent years driving and walking the French countryside, exploring the megalithic landscape, and ended up discovering a huge ancient system of concentric oval-shaped dykes centered (along with the aforementioned road system) on Sens--a massive terraforming project that is still reflected in existing roadways and the course of major rivers, as well as the location of towns. Besides standing stones, the region is also full of Merc- place names (Mercury...marker-y), hinting at its megalithic importance. Mestdagh argued that Sens had not only been the French megalithic capital (which Coppens adds, is probably why Caesar stationed his army there; it had been the home of the Celtic tribe Senones, "The Elders"), but that it had been the original Atlantis, having long ago been a kind of island surrounded by that dyke system--the region is still called Ile de France.
Whatever you want to make of the Atlantis (or Valhalla) part of the argument, that Sens had been hugely important to Megalithia and remained so even through the Middle Ages could explain certain oddities, including the curiously powerful title held by the city's archbishops, "Primate of Gaul and Germany." (Wiki also reveals Sens had the biggest and one of the very first early Gothic cathedrals.) A map of the Sens region and the odd arrangement of towns in the surrounding area is here: http://wiki.atlantisforschung.de/index. ... l_Mestdagh
There's more, but basically, many of Mestdagh's insights sound like they harmonize very well with yours (not that I needed further convincing), and his evidently largely-ignored discoveries seem to have been made in an authentic A-E spirit.
Eric