Property law comes partly from legislation — Acts of Parliament, and all the Rules, Regulations and Orders made under them — and partly from cases decided by the higher courts. In deciding cases, judges give rulings on questions of law. Sometimes a ruling is about the meaning of legislation; sometimes it is about the “common law”, meaning simply the law which has been developed by the judges themselves, over many years or even centuries. There are many series of law reports, in which key decisions are published. Nowadays, transcripts of judgments of the higher courts can be found on the website of the British and Irish Legal Information Institute: https://www.bailii.org/.
Some law is predictable: you could predict that there would be a law against murdering people. (There is.) Property law is predictable in broad terms: there is bound to be a system of law governing the ownership of land, and rights and liabilities over it. But the finer detail is not predictable, since there are many possible ways of constructing such a system of law.
The task of a legal adviser is to understand the legislation and case law, to explain carefully how it applies to a given set of facts, and to identify the best way forward.