The UK is a constitutional monarchy. The Head of State is the monarch (not a president), whose duties, functions and powers are conscribed by convention. One of the conventions is that the monarch is politically neutral.
England and Wales operate a common law system which combines the passing of legislation but also the creation of precedents through case law. The laws are established by the passing of legislation by Parliament which consists of the ‘Monarch’, the House of Commons and the House of Lords. The House of Commons is directly elected by the people and the Prime Minister is traditionally a member of this House. The Court System and case law are controlled by the judiciary which is completely separate to Parliament.
Books introducing the UK legal system can be found on Level 2, the entrance level to the library, with shelfmarks beginning KL11.
Online portals to reliable sources of UK law
Below is the Court Structure for the England and Wales (taken from HM's Courts & Tribunal Service website).