If you're looking for second-hand law books specific to UK government and legislation, there are several specialized sources and strategies to consider:
http://www.lawbooksellers.co.uk/
Online Marketplaces
2nd hand law books
Specialized Online Retailers
Academic and Legal Bookstores
University Bookstores
Online Classifieds
Student Networks
Libraries
Tips for Buying Second-Hand Law Books
By exploring these sources, you should be able to find a wide range of second-hand UK law books that meet your needs.