Lexis+ provides a Legal Glossary to help you understand the use of legal language in UK law and any other legal specific term you may need to know.
Westlaw has a section called Index to Legal Terms (this link will only work if you're already logged into Westlaw) which contains three dictionaries that provide multiple definitions, with varying detail, to help you understand UK law. The index also links and cross-references cases, legislation and journal articles if mentioned in the definitions.