YA novel: pocket-dimension prison on a floating rock, time moves differently, UK contemporary setting. Read 2013-2014. Spoiler.

[Cameron]

Read in the UK around 2013–2014. Contemporary vibe, teens had phones. Opens near a skatepark. A boy is sentenced by a shadowy authority to a prison in a pocket universe, basically a floating rock. The prisoners are mostly teens and don’t age while inside. Time outside moves slower, and when a sentence ends the prisoner returns to the same moment they left, so no one believes they were gone. There’s a warden figure. I remember an apple being given as a small act of kindness. The boy falls for a girl who turns out to be a mole for the authorities in exchange for a shorter sentence. In the end she decides to stay to enable an escape, and the boy chooses to stay with her. Final image is the two of them looking out over a desolate landscape, and the passage is closed, trapping them.
Not: Incarceron, Escape from Furnace, Darkside, TimeRiders, The Loop. Any ideas?

[Becca]

The Traitors by Tom Becker?