Prompted by James, I have reread "The Hollow Hills" and "The Crystal Cave", by Mary Stewart. These are historical novels of the time of King Arthur (shortly after the Roman withdrawal from Britain), aiming at a sort of realism, except for a small amount of supernatural assistance given to Merlin, most usefully as precognition. Merlin sees his visions and occasional coincidences as gifts from a God. He holds that the different Gods worshipped at this time are different faces of one God. By initiation, and after a youthful epiphany, he follows Mithras himself. Merlin does not follow a long term plan he has sketched out, but the hints dropped by his God. Perhaps in a hard SF setting we might have a similar combination of foreknowledge and arbitrary instructions if he was working for an intelligence agency. There's not a lot of action packed into these books, but I found the setting, and the sparing but effective supernatural nudges, fascinating. I am not as mobile as I would like now, but I have previously been inspired to visit some places claiming arthurian connections. Most recently I watched some youtube videos on Mithraism - explained as everything from an excuse for a dining club to a reaction to the discovery of the precession of the equinoxes.

PS - the legend of Arthur still resonates, at least by inspiring mistakes - we re-opened the "King Alfred Church Hall" recently, so named from the best claim to fame that can be found in the history of our pleasant but undistinguished market town. The first few leaflets advertising this advertised the "King Arthur Church Hall"