She did write a handful, most of them set later than the Roman Britain stories we focus on in fandom. (*No guarantees that it's the protagonist or the main POV character.) It seems to me that most of these remain fairly obscure even among fans, so if you're looking for this sort of thing:
- The Queen Elizabeth Story (1950, children's novel)
- The Armourer's House (1951, children's novel; Tudor period)
- The Shield Ring (1956, young adult Dolphin Ring series novel; Norman conquest)
- Lady in Waiting (1957, adult biographical novel; Tudor/Stuart)
- The Rider of the White Horse (1959, adult biographical novel; English Civil War)
- The Chief's Daughter (1967, children's short story; Bronze Age)
- The Flowers of Adonis (1969, adult biographical novel, two of the several narrators; Peloponnesian War)
- "The Man Who Died at Sea" (1970, ghost story anecdote)
- "Flowering Dagger" (1977, young adult short story; Bronze Age)
- Shifting Sands (1977, children's short story; Stone Age)
- Tristan and Iseult (1978, legend retelling; Arthurian)
- Flame-Coloured Taffeta (1986, children's novel; 18th century IIRC)
Honourable mention: Song for a Dark Queen (1978, [ostensibly] young adult novel, is about Boudicca but told through another narrator)
Have I missed any? Have you read these? What did you think of them?