Why can't the U.K build more prisons?

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I was thinking yesterday about the dire situation the U.K is in with regards to the snetances handed out to criminals when they are in court. I remember reading that judges actually have targets of how many people they should be sending to jail and how many spaces are left. I seem to recall they were advised a while ago not to send people to jail for more minor crimes as there was not enough space! I'm sure I'm not alone in thinking that it would be best to build another 10 prisons that could hold 5000 prisoners each. That's 50,000 more places and hundreds if not thousands of jobs created. The government already has billions of pounds stored away in case of emergencies so would easily be able to fund it in the first instance. I would happily pay more tax if offenders were brought to justice and handed sentences that reflected the crime and/or acted as a deterent. Wouldn't it be nice if someone who was sentenced to 20 years in prison wasn't out after 8 years for good behaviour. What if you spent 6 months in jail for causing anti-social behaviour like grafitti and riding illegal motorbikes. We could actually lock people up for 5 years for possesing an offensive weapon instead of just talking about it. I think people see England as a soft touch. I can't remember the last time I looked at the news and saw a fitting sentance for a convicted criminal. It's about time something was done about overcrowded prisons so we can get these people locked away for good. What are other people's views on the state of our justice system?