http://www.sentencing-guidelines.gov.uk
This site contains copies of all final guidance from the Sentencing Guidelines Council along with advice from the Sentencing Advisory Panel.

http://www.justice.gov.uk
The Ministry of Justice site gives details of Statistical Bulletins, Statistical Findings, Research Studies, Occasional Papers and Research Findings. Although some of them are short and summarise recent findings, some are very detailed and run to several hundred pages. Information from before May 2006 can be found on the Home Office site <http://www.homeoffice.gov.uk>.

http://www.magistrates-association.org.uk
The vast majority of offenders are sentenced by magistrates. This user-friendly site explains what they do, what powers they have, and, if you are interested, how you can become a magistrate yourself.

http://www.cjsonline.gov.uk
There are two reasons why this site is particularly useful: it gives information on all aspects of the criminal justice system, including sentencing; and it provides an excellent list of hyperlinks to other criminal justice sites.