A rational, evidence-based criminal justice system would always prioritise rehabilitation. Over and above the moral imperative to "save" the individual from a self-destructive pattern of behaviour (most criminals are fuckups, not careerists), a pragmatic calculation is that you can most effectively cut crime by reducing re-offending rates, as the majority of crimes are committed by repeat criminals. However, the justice system tends towards the punitive because it remains at heart a pillory, a means by which we collectively identify and deplore bad behaviour, though "we" in this context inevitably biases towards the attitudes and interests of the powerful. This explains why a rather ineffectual protest by a hipster dilettante against the Boat Race will land you 6 months in jail, while an assault on Chris Kirkland by a convicted hooligan will only get you 16 weeks. Chris is not yet a national treasure, hence the lower tariff.
Back in the day of the death penalty, the public spectacle of punitive justice tended to focus on personal morality and the well-crafted dramas of human fallibility. Ruth Ellis died not simply because she was convicted of murder but because she was "that sort of woman". It's perhaps not entirely coincidental that the end of capital punishment (suspended in 1965 and abolished in 1969) occurred shortly after a shift in attitudes towards personal morality, marked by the Profumo affair in 1963.
Today, the punitive spectacle of the law is more likely to focus on group behaviour, despite the best efforts of the tabloids to demonise individuals (witness the current Savile-related headhunt). The draconian sentences handed down after the 2011 riots were an example of this desire to punish "the lot of them". The decision of an Italian court to sentence scientific experts to jail for failing to correctly forecast the L'Aquila earthquake is an extreme form of public retribution. Some of the scientists have drawn self-glorifying parallels with Galileo, but the real echo is of the Roman's sacred respect for auspices. Predicting the future well-being of the people is a serious business and has no place for the nuances of probability or uncertainty (a worrying thought for economists). Given the theatrical nature of Italian law (the real verdict and punishment is always decided after appeal), the auto da fe is likely to be downgraded to a slap on the wrist. Italian politicians will not want the scientific community, or experts generally, to go on strike.
It is in this theatrical light that we should view David Cameron's recent speech on punishment. The core of it is a commitment to rehabilitation, partly for the utilitarian reasons outlined above and partly to provide a priori justification for cuts in spending. Neoliberalism sees prisoners as a wasted asset that imposes a financial cost on the state. While the dream of making prisoners pay remains attractive, the reality is that keeping them out of prison, and part of the reserve army of labour, is the most efficient strategy. The problem, for a politician, is squaring this with the demand for retribution. The solution is to simultaneously claim that prisoners have a cushy life and demand that they are made to suffer, hence such tokenistic gestures as opposing votes for them.
Perhaps the most bizarre spectacle proposed by Cameron is the upgrading of tagging to the use of GPS, which will allow "offenders to be tracked by satellite 24-hours a day". This is the constant surveillance, leading to self-repression, of the Panopticon and Big Brother. It's also an update on the old TV detector van nonsense, in which no one appears to be able to distinguish between a transmitter and a receiver. TVs, as receivers, do not transmit a signal, so they cannot be detected. All the inspectors do is cross-check the database of licence addresses against houses, on the assumption that almost everyone has a TV. The old tagging technology is similarly a triumph of theatre over reality. The tag is a passive device that must be held against a transponder that is in turn activated via the offender's home phone line (yep, an analog modem is involved). Anecdotal evidence is that the monitoring is infrequent. You might only get one call in an evening, after which you could happily go down the pub. In addition, the transponder could easily be "accidentally" broken or unplugged. This might produce an alert, but that wouldn't be evidence of a breach and wouldn't justify sending out a squad car.
GPS is similar, insofar as satellites don't track anything (spy satellites take high-resolution photos, which is another kettle of fish). A GPS device can calculate it's own geo-location by polling the transmission of three satellites, assuming you're not underground or your line of sight isn't obscured by tall buildings. Or trees. Or hills. To enable monitoring, the device must then transmit a signal to a central receiver, which it would probably do via a cell network, which means it helps if you live in town. 24-hour monitoring will also require a pretty decent battery, which you the offender must remember to regularly recharge. In other words, we're talking about equipping offenders with something that has most of the capabilities of a smart phone (cushy) but doesn't do voice calls or text (punishment).