@Bromley boy that's almost completely arse-backwards (pardon the phraseology I I can't think of anything else that fits).
The purpose of a custodial sentence is not just punishment (and the punishment aspect is
only deprivation of liberty) or to act as a deterrent, they're just small parts of it. The principal things are to separate people from the circumstances that led to their offending and to provide support, training and education so that upon release they can become a productive member of society. However, the combination of grossly underfunded and overcrowded prisons, grossly underfunded probation services, undermining of the Rehabilitation of Offenders Act (and similar) and the failing social security and benefits system mean that the system
cannot do this job and far too often people are left with no option but to break the law. Every time that a prisoner re-offends after release it means that the prison system failed - doing the same thing over and over again and expecting a different result is not a sign of intelligence.
What prison you are sent to, and where and how you are housed within that prison, needs to be assessed on the combination of many factors, including:
- The offence you committed (e.g. don't surround a young pickpocket with experienced burglars)
- The risk you pose to the general pubic
- The risk you pose to prison staff
- The risk you pose to other inmates
- How likely it is you will abscond or (try to) escape
- The support you need (medical, psychological, emotional, educational, etc)
- The training programs available (e.g. if someone wants to learn a trade, say plumbing, then it's no use sending them to a prison that has academic type training only)
All prisoners, need to be given things to do - they need access to books, to the television, to (supervised in most cases) internet access, training, education, etc. Locking someone up for extended periods with nothing but the walls to look at will do nothing except make their mental health worse, make their reintegration into society that much harder and therefore make them more likely to re-offend - chances are they have no idea how to do anything else and even if they do the lack of support outside means they have no way of supporting themselves while starting to learn how to put that in practice. Allowing prisoners a means to make money while inside is also good, as this will help them find their feet when released and teach them the benefits that come from earning a living, teach them about managing money, etc. It can also help fund the prisons - if a prisoner earns enough that they would pay income tax on the outside then just pay the exact same percentage to the prison.
Even someone who is going to be locked up for 20+ years needs all this as it means that they are less likely to attack prison guards and/or other inmates and will give them the benefits when they are released.
Very short, or part time (e.g. weekend only) prison sentences can work in a few cases but only where the person concerned already has the skills and aptitude to operate in society and only needs the consequences of their actions enforcing. In any other case they are are pointless waste of money.