Past undergraduate dissertations can be found on the dissertation module's reading list
The Internet Journal of Criminology also publishes Masters and first class undergraduate dissertations in criminology from UK and other universities
The UK Data Service is a really good source of existing data, which can be useful if you are unable to collect data yourself.
Check out their student pages, specifically designed for those embarking on dissertations. They cover finding, using and analysing their data via articles, videos and tutorials.
A good introductory video is: Dissertation projects: introduction to secondary analysis for qualitative and quantitative data. They also run this live, check out their events pages for upcoming workshops.