"Natural law" was & is used in ethical and political thinking as well as "narrow" legal philosophy. Consequently, SOLO searches may well reveal that the Bodleian Philosophy and Theology Faculty Library in particular holds a lot of useful material, as well as the Reading Rooms in the Old Bodleian & the Bodleian Social Science Library.
Medieval scholars benefited from the rediscovery of the classical tradition - Plato (via St Augustine) Aristotle & Cicero. As saved Christians they sought to reconcile these older principles with Roman Catholic teaching, revealed Faith with Reason.
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)
Other big names in the development of natural law in its widest application up to the (18th are:
Searches to use in SOLO could be
Below are just a sample of results, starting with those in the Law Bod. The monographs with shelf marks starting Jurisp are on Level 2, the entrance level, to the Law Bod.
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 for example
Natural law - history
or the names of the leading thinkers or bases of the contemporary thinking such as Aristotle, Aquinas.
The Law Bod's collection of works on legal philosophy and theory is have shelf marks beginning Jurisp (abbreviation for Jurisprudence)
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.