What is the difference between a Bill, an Act and a statutory instrument?
They are three stages and forms of law-making, and confusing them leads to reporting that says something "is now law" when it is not.
A Bill is a proposal for legislation. It has no legal force. Bills pass through both Houses of Parliament in stages — first reading, second reading, committee stage where it is examined line by line, report stage, and third reading — then repeat the process in the other House. Amendments pass back and forth in what is informally called "ping pong" until both Houses agree.
Types of Bill: Government Bills, which make up most legislation; Private Members' Bills, introduced by backbenchers, of which very few become law; and Private Bills, affecting specific organisations or localities.
An Act of Parliament is a Bill that has completed all stages and received Royal Assent. It is now law — with an important caveat below.
A statutory instrument (SI) is secondary or delegated legislation. An Act frequently grants ministers power to make detailed rules later, and SIs are how that power is exercised. They set out the specifics: rates, thresholds, procedures, technical schedules.
There are far more SIs than Acts — thousands each year against a few dozen Acts — and they receive dramatically less scrutiny. Most pass under the negative procedure, becoming law unless Parliament objects within a period, which almost never happens. A minority use the affirmative procedure, requiring approval. This imbalance is a long-standing constitutional criticism, particularly regarding Henry VIII powers, which allow ministers to amend primary legislation by SI.
The caveat about Acts. Royal Assent does not mean the law is in force. Many Acts contain commencement provisions bringing sections into effect on later dates, by commencement order. Some sections are never commenced at all, remaining on the statute book unused for years.
Scotland, Wales and Northern Ireland have their own legislatures and equivalent processes.