What is legislation?
There are two types of legislation:
Primary legislation: Acts of Parliament (Statutes) which passes through parliament as Bills (draft legislation). When both Houses of Parliament have approved the bill it will receive Royal Assent and become an Act. There are two types of acts, Public General Acts and Local and Personal (also called Private) Acts.
Secondary legislation: Statutory Instruments, Orders or Rules. Also known as secondary, subsidiary, subordinate or delegated legislation. They allow provisions of an Act to come into force or amend details of an Act.
You can find Queen’s Printers copies of statutes or statutes as originally enacted online in Justis; more recent material (from 1988-) is freely available on Legislation.gov.uk . As well as legislation as enacted Legislation.gov.uk also includes revised versions but the most reliable online sources for up to date legislation is LexisLibrary and Westlaw.
Finding legislation by title
A really good place to start is entering the title, e.g. Road Traffic Act 1988 into JustCite (below) to check availability as this will point to Westlaw and LexisLibrary. For Statutory Instruments you can search by title or SI number e.g. 2015/1478 this is the year followed by the SI number.
Both LexisLibrary and Westlaw provide amended Acts and Statutory Instruments (SIs). As well as the text of the legislation they also provide information about the clauses including what revisions have been made.
Finding legislation by subject
Since English law is a common law system, finding out what the current law is isn't necessarily a matter of looking at statutes or Acts of Parliament.
To find out what the current law is, start by looking at a major text or encyclopedia. Halsbury’s Laws is the pre-eminent publication - find it via LexisLibrary. Every statement in Halsbury's Laws is backed by case or statutory authority, so it's a very good place to start research.