@WTSTC Thank you for that contribution. You make some valid points. Is Prince Charles a Druid? Certainly Winston Churchill was and Queen Elizabeth is. I can’t remember which Orders they belong to, I think it may have been the AOD Ancient Order of Druids. This Order, along with a number of others, were or are friendly societies where members look after each other and sometimes donate money to charitable causes. It is entirely possible that Prince Charles belongs to such a group. We might not know for sure, but it seems like reasonable speculation. For the history of the AOD see Ronald Hutton, including The Druids, Blood and Mistletoe and Witches, Druids and King Arthur. Ronald goes into this time period quite intensively. But this does not prove Prince Charles is one of us, this is still speculation. The AOD are from what I know different from other societies such as the Masons, as were the other similar Druid friendly societies. The difference between them and a normal friendly society would be the ritual elements and observances at important places such as Stonehenge. Don’t forget the Welsh Gorsedd, probably a number of important people are connected to that as well; the connection may be with the Welsh or with culture or it may have spiritual dimensions.
The Celts covered a large area, England, Scotland, Wales, Ireland (Cornwall), in order to maintain religious functions the number of Druids would have had to have been seriously large in order to cover all of their functions. We know that the Celts and the Romans did interact religiously as well as in commerce; many Celtic gods were combined with Roman gods in places of worship, for example Mercury and Rosmerta. Did Roman citizens go to Druid ceremonies? An interesting question. Some may have done. I have not come across any proof of this so far but it is a plausible idea. Possibly the gods were combined to keep everything and everyone in one place, as it were.
Greywolf your arguments make sense, I think that this is a very credible scenario. It appears that Anglesey may have been given undue emphasis as an account, to the point where one might think that all Druids were massacred there; as you point out, this makes no sense when you stop to think about it.
Does the severed head cult relate to the stories of wondrous heads? Bran for example, of course. Which would mean that the Mabinogion takes on more importance if it is indeed the main survival of some of our past beliefs.