Despite the fact that he could read and write, he was still, by class, a commoner. By the 15th C more middle class were literate than you might expect. The great Geoffrey Chaucer lived just before this time (c. 1343 – 25 October 1400) and he was very literate, though he was a tradesman's son. He managed to rise high in the ranks of the court, being in the employ of Prince John of Gaunt, and acting as a spy for Edward III (due to his skill with French). He married one of the wards of Queen Philippa, Philippa de Roet, whose sister caused a scandal by becoming first the mistress, then wife of Prince John of Gaunt (and mother to four of his children), but this was all well recorded. Had a Plantagenet daughter, even an illegitimate one, married a man without a title, it would have been written down. Once again a generation or so before your time, one of the highest non royals in the country, 12 yo Matilda de Brocas ran off with 14 yo John Foxley, whose father, though the architect for Windsor Castle and a landowner, was nevertheless a commoner. The two lovebirds caused a firestorm that is well recorded. Only the fact that they had consummated the marriage by the time they were caught made her father accept the marriage, but he went for the priest who performed the ceremony.