Generally, the senior judiciary sit in the senior courts (the High Court, the Court of Appeal and the Supreme Court) and all other judges sit in the lower courts (the Magistrates’ Courts, County Court and Crown Court).
The Constitutional Reform Act 2005 made important changes to the way in which the judiciary is run. While the judiciary has always been independent (of government and the legislature), the Act placed that independence on a statutory footing to ensure that judges remain free from political influence. Amongst other things, it created the Judicial Appointments Commission. This is an independent commission which selects candidates for judicial office in courts and tribunals in England and Wales. It is responsible for ensuring that candidates are selected on merit, through fair and open competition, and from the widest range of eligible candidates.
The Constitutional Reform Act 2005 reformed the office of Lord Chancellor and established the Lord Chief Justice as head of the judiciary of England and Wales. It also established the Supreme Court. Before this, the most senior appeal court had been the House of Lords which, of course, is also part of the legislature. Moving this appellate function into an independent Supreme Court has ensured that the legislature and the judiciary are truly separate (as required by the doctrine of separation of powers).