5 hour exam - not allowed water?

I just found out one of the exams I’m meant to be taking (sqe) is a 5 hour exam and you are only allowed water after 2.5 hours. This really has worried me. If I don’t drink water often I get a bad headache. I also can’t focus and water helps me calm down. I don’t know why this is a rule but do you think I am able to get it changed?

Don't worry. When you enter the exam hall inform a invigilator about it. And hopefully, they'll let you drink whenever you prefer.

Lots of people that have already taken the exam have confirmed that no refreshments are allowed and are locked in your locker