Traditionally, the defining principle of the British constitution was parliamentary sovereignty.  As Parliament's website puts it, this doctrine "makes Parliament the supreme legal authority in the UK, which can create or end any law".

The doctrine of parliamentary supremacy goes back at least to Tudor times, and was upheld by Sir Edward Coke and Sir William Blackstone.  However, the older authorities are not unanimous in affirming it, and it has been somewhat diluted in recent times, particularly in the context of the UK's membership of the European Union (albeit some Eurosceptics' fears in this regard seem a little overblown).  There is a reasonably strong case to make that the sovereignty of Parliament is no longer absolute.

Acts of Parliament, or statutes, are made jointly by the three components of Parliament: the Queen, the House of Lords and the House of Commons.  Parliament has been legislating for a long time, and some very old statutes remain on the books.

In practice, the Commons is the overwhelmingly dominant part of Parliament.  The monarch has not withheld assent from a piece of legislation since 1708 (though there are cases of the royal assent being granted incorrectly).  The House of Lords is subordinate to the House of Commons: this subordination was already acknowledged by the late 19th century, and can be traced back to the post-Cromwellian period.  Under the Salisbury convention, the Lords never rejects Government bills that have been presented to the voters in an election manifesto.  The Lords can reject other bills, and the Commons can force such bills to be passed, but these are both very rare eventualities.  To a large extent, the monarch and the Lords form what Bagehot called the "dignified" rather than the "efficient" parts of the constitution.

Reform of the House of Lords has been a live political issue at various times over the last century.

The House of Commons is elected today by universal suffrage using single-member geographical constituencies and the first-past-the-post electoral system.  The franchise remained very restricted until the 19th century, when a series of Reform Acts were passed, in the face of significant opposition (notably, the 1832 Reform Act and the 1867 Reform Act).