A question was recently asked about types of UK legislation.
An Act of Parliament creates a new law or changes an existing law. Acts are known as ‘primary legislation’ because they do not depend on other legislative authority. All Acts start as Bills which are introduced in either the Commons or the Lords. When a Bill has been agreed by both Houses of Parliament and has been given Royal Assent by the Monarch, it becomes an Act. An example of an Act is the Health and Safety Work etc Act 1974. An Act may come into force immediately, on a specific future date, or in stages. Future changes to the law happen through the passing of another Act or delegated legislation.
Statutory Instruments (SIs) are known as secondary legislation, which is law created by ministers (or other bodies) under powers given to them by an Act of Parliament. Secondary legislation is also known as ‘delegated’ or ‘subordinate’ legislation. SIs are considered by The Joint Committee on Statutory Instruments (JCSI). The role of this committee is to ensure SIs are legal and do not go beyond the powers specified in the parent Act. Once scrutinised SIs are then ”laid” before Parliament for final approval. https://www.parliament.uk/about/how/laws/secondary-legislation/statutory-instruments-commons/ An example of an SI is SI 1999/3242 The Management of Health and Safety at Work Regulations 1999.
Various guidance documents support legislation. Some, for example The Approved Codes of Practice (ACOP) issued by the HSE under the Health and Safety at Work etc Act, have a special legal status. https://www.hse.gov.uk/legislation/legal-status.htm.