I’m not entirely sure whether there are extra writing booklets for the short answer section, but there are definitely extra writing booklets available for the writing section.
I think that writing beyond the space given should be fine, as long as it's within the boxed margins.
Since the short answer section doesn't state "Extra writing booklets are available", I think it would make sense that there aren't extra booklets available, because "The lines provided give an indication of the suggested length of student responses."
The only concern is that if a student completely messes up their answer, implying the need for new writing space. However, I would assume this is not common since most people are prepared as they commence their exams and think of points to include in their answer before writing it.