TCTM04119 | Income: Employment income rules: Payments made to a person serving a custodial sentence or remanded in custody awaiting trial or sentence, for work done while serving the sentence or remanded in custody

The Tax Credits (Definition and Calculation of Income) Regulations 2002, Reg. 4(1)(m)

This refers to payments made to people who are serving a custodial sentence or who are remanded in custody for the work they actually do whilst they are serving their custodial sentence or remanded in custody.