>why would a 'priesthood' move without their population/community/followers?<

It's mostly speculation Moth (which is why it's so enjoyable :-) but there <i>are</i> later examples of just a handful of 'priests' going a long way to convert the 'natives' - and going without their population, community or followers. I'm thinking of the introduction of Buddhism to Japan by a few Korean monks; St Columba and his meagre band of twelve companions who sailed from Ireland to Iona - a mission which was to play a leading part in the conversion of Britain to Christianity. Just two examples of a small party of priests on a mission. Perhaps something similar was going on between Preseli and Salisbury Plain - who knows. Again, it's all speculation, another idea for the pot, and I'm happy for it to be shot down if it doesn't make sense.