thalbergmad, you need to find out whether you have a paid or unpaid meal break. As far as I know, if you have an unpaid meal break you are allowed to leave the premesis, otherwise you must remain on site.
This is Part of EU legislation
https://www.incomesdata.co.uk/information/worktimedirective.htm#Article3Article 4 - Breaks
Member States shall take the measures necessary to ensure that, where the working day is longer than six hours, every worker is entitled to a rest break, the details of which, including duration and the terms of which it is granted, shall be laid down in collective agreements or agreements between the two sides of industry or, failing that, by national legislation.
https://www.opsi.gov.uk/si/si1998/19981833.htm#10Rest breaks
12. - (1) Where an adult worker's daily working time is more than six hours, he is entitled to a rest break.
(2) The details of the rest break to which an adult worker is entitled under paragraph (1), including its duration and the terms on which it is granted, shall be in accordance with any provisions for the purposes of this regulation which are contained in a collective agreement or a workforce agreement.
(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.
(4) Where a young worker's daily working time is more than four and a half hours, he is entitled to a rest break of at least 30 minutes, which shall be consecutive if possible, and he is entitled to spend it away from his workstation if he has one.
(5) If, on any day, a young worker is employed by more than one employer, his daily working time shall be determined for the purpose of paragraph (4) by aggregating the number of hours worked by him for each employer.