OU modules (with a couple of exceptions) span a 9-month academic year. There's no real concept of terms/semesters.
You do get scheduled break weeks, and even outside of those you should be able to make those holidays work as long as you plan carefully. (Not near exams, ideally not near group projects, check assignment deadlines, get ahead beforehand or make a plan to catch up after, etc).
As for actively studying while away without internet, that'll depend heavily on the specific course- e.g. for maths it'd usually be fine, but it'd be impractical for most computing modules.