Karl Marx wrote:
We've been steadily increasing our use of imprisonment over the last 20 years or so. We're currently at about capacity at just over 80,000. I remember when as a student (a little over 10 years ago) a prison population of 45,000 was regarded as a scandal and a crisis.
The last couple of years has also seen the implementation of a new indeterminate sentence for dangerous offenders. When implemented, government didn't think it would be that popular, but it has been! It's effect is simple. For most sentences, the judge simply gives a sentence, and that's what you'll serve (with the usual period of being released on license). The new sentence is indeterminate. You serve a tariff that represents the harm caused by your offence and only then are you eligible for release. However, they don't just kick you out; you have to convince the parole board that you're a manageable risk. The courts have really gone for this sentence and there are loads in our prisons that would have been released by now under the old determinate sentences. Additionally, because our prisons are at capacity, it's more difficult to finish educational programmes, and therefore more difficult to convince a parole board that you are a manageable risk, and it's therefore more difficult to get out, thereby contributing to the over crowding! Parole boards are also now over worked and there are therefore delays in hearing applications for release.
In short, it's all the fault of the gutter press!

To summarise.... having bowed to the "lock them up and throw the key away" lobby, they've now found that they need a lot more keys that they don't really have the political will to provide?
"Dear Sir, having filled our prisons we have now decided to build a new one at the bottom of your road. Hope that's ok."
"Public Enquiry! NIMBY! Why don't we just hang the lot of em, we never had this problem when we had National Service...."