St. Helena, as Waugh occasionally notes, is a good subject for a novel: there is a definite historical framework, a fair amount of legendary material of varying and difficult-to-untangle degrees of historicity, and a lot of unknowns. Waugh doesn't pretend to be doing any more than giving us a novel, although, outside a few liberties he explicitly mentions in the preface, he avoided anything that directly contradicted known historical fact or could not receive some support from something in the tradition.
The edition of the book I have has a portion of a 1960 interview, in which he explains what he saw himself doing in the work:
The fact of the True Cross was that there was an actual piece of wood, a historical fact, behind the Gospel. Whether or not the wood she found was the Cross is open to doubt, but at that time all those Asiatic cults, the Gnostics and people, were trying to theorize and symbolize and fine away the simple facts of an actual crucifixion on a piece of wood; and she I represented as being a simple English girl thrown greatly to her disgust into the imperial life, not the least enjoying her high position, and putting her finger at once on what was wrong with imperial Rome at that time, which was that they were losing the sense of actuality. That you might indeed say was a didactic book.
Evelyn Waugh, Helena, Little, Brown and Company (New York: 2012).