Law is an academic degree at undergraduate level. There’s no expectation that your PS should include work experience (and often PSs that are too focused on work experience are demonstrating a lack of understanding of what the degree involves).
Your PS should be 80-90% on academic topics and activities linked to law and legal aspects of current affairs. If you’re not sure what to write about try listening to law in action on bbc sounds and find some topics that you find interesting and then find out more about them and write about that interest.