They totally exist. Anyone between 15-21 can be placed in a Young Offender Institution. Anyone 18+ can be placed in an adult or young offender. And anyone over 21 has to be in an adult prison.
In this scenario they are all sent to the same prison that has a special block for "foreign prisoners". This block isn't supposed to hold certain types of prisoners, like sex offenders, but according to the article it does.
So there are lots of steps that are failing, why this is rather "shocking".