[Peter Knutsen]

Often pagan practices were tolered by the Church, in the later medieval period - or any area that had been converted for several centuries, as long as the people still participated in Church activities. I think only outright worship got slapped down hard (since that often included sacrifices of produce, animals or humans, or interacting with an idol, or several of them), and perhaps (but not necessarily) attempts to practice magic.

1300-or-so AD Norman England, in which Worminghall is set (as far as I know), certainly counts as having been converted for several centuries, so there's no reason for the Bishops to have a hard-ass zero-tolerance attitude. And the practice of magic obviously has to be looked at in terms of fitting into the setting, since it's not standard medieval, but rather a place where magic is acceptable enough to have a school devoted solely to that subject.

As for the Keltic gods, many of them, if not all, were co-opted by the Keltic Church and/or Catholic Church and turned into saints, with the goddess Brigid being the best known case. So that's one way for the Druids to sneak in a lot of worship without being caught at it - they can just be very devoted to Saint Brigit, and so forth.