The law of England and Wales (but not Scotland and the other sections of the UK) is considered by some to be one of Great Britain's great gifts to the world. Also known generally as the common law (as opposed to civil law), it was exported to Commonwealth countries while the British Empire was established and maintained, and persisted after the Bristish withdrew or were expelled, to form the basis of the jurisprudence of many of those countries. Actually part of the English legal system has always been considered to be based upon the civil law, namely the ecclesiastical courts and the courts of admiralty.
The essence of common law is that it is made by judges sitting in courts, applying their common sense and knowledge of legal precedent to the fact before them. Because common law consisted of using what had gone before as a guide, common law places great emphasis on precedents. Thus a decision of the highest court in England and Wales, the House of Lords (the judicial members of which are referred to as Law Lords) is binding on every other court in the hierarchy, and they will follow its directions.
It is also for this reason that there is no Act of Parliament (the normal method for creating laws in the UK) making murder illegal. It is still a common law crime - so although there is no written Act passed by Parliament making murder illegal, it is illegal by virtue of the constitutional authority of the courts and their previous decisions. Murder carries a mandatory life sentence.
However, while England and Wales retains the common law the UK is part of the European Union and European Union Law is effective in the UK. The European Union consists mainly of countries which use civil law and so the civil law system is also in England in this form, and the European Court of Justice, a prodominantly civil law court, can direct UK courts on the meaning of EU law.