Some subject searches to try in SOLO
Law -- Philosophy -- History
Law -- Philosophy -- Europe -- History
Jurisprudence -- History
Law - interpretation and construction - History
Law - moral and ethical aspects - History
More specific searches could be the names of the leading thinkers.
The Law Bod's collection of books on legal philosophy and theory is have shelf marks beginning Jurisp (abbreviation for Jurisprudence) It is shelved on Level 2, the entrance level to the library.
Other Libraries in Oxford
If your interest is in this area, SOLO searches may well reveal that useful material is also held in the Bodleian Philosophy and Theology Faculty Library , the Bodleian Social Science Library, the Sackler Library , as well as the Reading Rooms in the Old Bodleian and Radcliffe Camera.
Natural law is not a concern restricted to lawyers. Consequently SOLO searches may well reveal that the Bodleian Philosophy and Theology Faculty Library in particular holds a lot of useful material, but also the Reading Rooms in the Old Bodleian & the Bodleian Social Science Library.
The Summa Theologica of St Thomas Aquinas is the great medieval contribution to the history of natural law theory. (Strictly legal aspects are largely in ST I-II, qestions 90-7)
Searches to use in SOLO could be
Below are just a sample of results, starting with those in the Law Bod. Books with a shelf mark starting Jurisp can be found on Level 2, the entrance level, to the Law Bod.