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.