Finding vacancies depends on the specific area you want to work in. Trainee Probation Officer roles are advertised through Train to be a Probation Officer and Probation Services Officer roles are advertised by each regional Community Rehabilitation Company. Each police service sets its own recruitment and selection policy, and most prisons recruit directly. Try some of the resources in suggested links. Networking is a good way to build contacts and find out about opportunities. You can also search for vacancies on UniHub. More advice is available in Finding a Job.
Employers will expect you to have some practical experience of the criminal justice system, working with people affected by issues of crime, or other vulnerable groups of people. So, you need to consider whether you want to work directly with people on the 'frontline' of services.
Gaining Work Experience
Most employers look for applicants with work experience, whether it is paid or voluntary. Look for opportunities in youth and community work, youth offending teams, mental health services or substance misuse services. Working with adults in an advisory or education related capacity would also be helpful. You may even find graduate opportunities through the organisations you work with.
Current Issues in the Sector
Employers expect graduates to keep up to date on the latest issues affecting the sector. To keep abreast of the issues affecting your chosen sector read the sector specific pages of the broadsheet newspapers and refer to the professional associations. As well as advertising vacancies, professional journals can help you to keep up to date with the industry news and can even help you to identify organisations to whom you could make speculative job applications. Try the British Journal of Criminology, British Journal of Community Justice and Probation Journal.
For some occupations you will need to do further study, examples include Social Work, Teaching or taking a Law Conversion course. Look at the relevant subject pages and postgraduate study section.