The National Archives have made available British Cabinet Papers covering the years 1915 - 1978. These can be searched by keyword / phrase and date. It is also possible to browse content by theme.
One of the most pertinent sections for lawyers is entitled 'Law, Liberty and Society'. Within it are documents discussing the criminal justice system (including the abolition of the death penalty), immigration and the Wolfenden Report.
Approach with care, I spent more time than was strictly necessary in the preparation of this post reading an array of fascinating papers covering a whole spectrum of issues.