Yes, it’s legal. And the 20 mins can be unpaid too.
It’s the bare minimum, but sadly the reality for many workers - not just young people in temporary summer jobs.

Does anyone know what the legal requirements or guidelines are for work breaks. My niece is staying with us whilst she does some summer hospitality work and she seems to be working 10 hour shifts with one 20 min break? Is this legal? She is nineteen.