You can debate all day on whether prisoners should be in prison and whether or not it rehabilitates them - however I don't think that offering a paedophile, a murderer or a drug dealer £5 an hour for sweeping up and cooking dinner is going to turn them whiter than white.
Prisoners belong in prison in my opinion, they are of great cost to society just for housing, feeding, cleaning and guarding them. Their criminal acts put them their and they owe a debt to society. Them being there is costing society a great amount so why should we then pay them and treat them well? Uk prisoners have an easy life in comparison to other countries. We all know some prisons mistreat prisoners, but the fact that prison service can be sued for not providing a working toilet shows just how much the prisoners know they can get away with.
Here's an idea, if you want paid for the work you do, get out of prison, get a job, pay your taxes, contribute to society again and that's rehabilitation, and your being paid for it.