Many thanks for your patience, I am pleased to be able to continue assisting with your query now. First of all, I am sorry to hear about the issues you have experienced in your situation.
Your legal rights to rest breaks at work are governed under the Working Time Regulations 1998. The particular section about the rest break entitlement is here:
The relevant part is this:
“the rest break provided for in paragraph (1) is an uninterrupted period of not less than 20 minutes, and the worker is entitled to spend it away from his workstation if he has one.”
So as you can see, you have the legal right to spend your rest break away from your workstation, which means away from the place where you work. That does not mean leaving the premises, but it does mean stepping away from the usual place where you do your work, be it a classroom, office, staff room. That could mean being allowed to walk around the grounds, or go to other places which are not ordinarily considered your workstation.
Hopefully, I have answered your query in a way that is simple and easy to understand. If anything remains unclear, I will be more than happy to clarify it for you. In the meantime, thank you once again for using our services.