The Regulations do NOT say folk can leave the premises. They say, "Rest breaks
12.—(1) Where an adult worker’s daily working time is more than six hours, he is entitled to a rest break.
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(3) Subject to the provisions of any applicable collective agreement or workforce agreement, 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."
The break can be away from the workstation, it need NOT be outwith the premises.