Westlaw UK contains the full text of acts since 1267 and selected statutory instruments since 1948. You can view historic and prospective versions of legislation, see annotations and find and links to case law, commentary and journals. Access this here: Westlaw UK.
Legislation.gov.uk carries most types of legislation and their accompanying explanatory documents. In some cases, only the original published (as enacted) version is available. You can access this here: Legislation.gov.uk.