‘Utilitarianism resources’ - The ‘utilitarianism’ website holds a wealth of material on this theme, including a number of sources relating to Jeremy Bentham and his theories of punishment:
https://www.utilitarianism.com/

The Sentencing Council for England and Wales and the equivalent body in Scotland set out principles and guidelines to inform the practices of the judiciary in deciding on what sentences to apply, and they also aim to promote better public understanding of the sentencing process:
https://www.sentencingcouncil.org.uk/
https://www.scottishsentencingcouncil.org.uk/

The Howard League for Penal Reform - an independent campaigning organisation which campaigns against the excessive use of punitive sanctions, and aims to achieve a fairer, and non-discriminatory justice system:
http://howardleague.org/