A poll published in Tuesday's Guardian apparently shows that most people feel that we have enough prison places and don't need to build more, and should find other ways of punishing people:

More prisons are not the answer to punishing criminals, says poll | Special reports | Guardian Unlimited:

A Guardian/ICM poll published today overturns the assumption that the public think tough prison sentences are the best way to tackle crime. It shows that a majority of voters think the government should scrap its prison building programme and find other ways to punish criminals.

Politicians in all parties routinely assume that voters think prison works. But 51% of those questioned want the government to find other ways to punish criminals and deter crime.

Of course many are in prison for offenses related to drugs consumption and the crimes many commit to satisfy addictions to substances that are artifically highly priced and because they are an illegal market. This illegal market itself creates more criminals and is the core of organized crime. Legalizing most of these substances would at a stroke enable us to empty the prisons of the hapless and hopeless addicts and enable them to voluntarily seek help knowing that they won't be treated as criminals for doing so, and if they didn't they could always manage their addictions more safely and affordably without resorting to crime to do so. And in the process, we'd free up prison space for real criminals.