No, students are only allowed to go through the exam paper during reading time. No annotation or any use of pen can be made until writing time starts.
Essentially, the aim of reading time is for students to read the questions (and perhaps have a quick look at the texts in the case of Paper 1) and based on their initial impressions of the exam paper, think of a potential strategy to complete it (for example, the questions that they should attempt first and/or questions that they may need to initially skip).
I hope this helps!