Uncial relates to a manuscript " written in uncials" according to Chambers. I was also uncertain as to the pluralised answer, but there is no logical alternative so I have stuck with it !
Hi samurite - I have just been reading Chambers - I think it is because it is the characters are more than one - uncial is one character :-/ I think. ????
I agree with JimJools. In the same way that if the clue was something like "a form of the Latin alphabet used in modern times (especially when filling in forms)" could be CAPITALS. That's plural too, yet the clue would suggest a singular answer.