Of course they will see it as punishment.
In prison there is not one minute's silence - day or night. They will live in constant fear of attack. They will suspect their food has been tampered with. They have lost their liberty. Visits are restricted. If they don't like their cellmates/neighbours - tough. They will not be able to relax/let their guard down. Their view of the sky will be limited.
No matter what a prisoner has done, no matter how heinous the crime, Britain is a civilised society and every prisoner has the right to food, a clean bed, medical attention, exercise. That is no compensation for lack of liberty and a life of fear.
Hopefully this pair will find their consciences and will have to live with the burden of their crimes, and the guilt and remorse, for as long as they live. That is the real punishment.