Most types of primary legislation (eg Acts, Measures, NI Orders in Council) on legislation.gov.uk are intended to be held in “revised” form, meaning that amendments made by subsequent legislation are incorporated into the text. Most types of secondary legislation are not revised and are held only in the form in which they were originally made.
A central criticism of legislation.gov.uk in the past has been that much primary legislation is not in fact up to date, in some cases lagging several years behind amending instruments. However, recent efforts have turned this situation around.
Legislation.gov.uk is now in the final stages of a programme to bring all the revised legislation fully up to date. Except for a few special cases that require extra work (such as the Taxes Management Act 1970), the backlog of outstanding effects will have been cleared and the revised legislation brought up to date by the end of 2018.