The 24-letter alphabet?

I'm reading Heresy by S J Parris which is set in 16thC England. In breaking a simple code the protagonist notes that 'there were twenty-four letters in the English alphabet'. Is this just a major error or might it have been true at the time? Does anyone know? Obviously I know that our alphabet was adapted over many years from the Greek and Roman ones and at one time did have 24 letters which were then added to but I have always believed that happened sometime before the 10th century and that there were 5(?) additions which were then eventually whittled down to the 26 letters we use today. Either way, I know of no indication that 24 letters were used in the days of Elizabeth I. Perhaps later in the book there will be some explanation but at the moment it just feels like a massive blunder that should have been picked up by the author or a proof-reader. (Incidentally, it's not a bad book at all or one of those that is full of typos so if it is a mistake it seems to be a solitary one, which is why I'm inclined to give the idea some credence.)

In the 16C, I and J, and U and V, would widely have been considered mere variants of the same letters.

#3 That does make sense. Thanks Andreas!