The Stationery Office
Although this is a commercial enterprise trying to sell you publications, it is still useful for tracking down bibliographic details, command papers numbers, etc.
For older materials you can use the HMSO/Stationery Office Annual Catalogues, which cover almost all official publications from 1920 onwards, located in the Research Reserve Social Science Reference Z 2009.E6
GOV.UK is the main hub for access to government information and services.
The publications page of the website lists material can be filtered according to department, date published, publication type and topic.
You can stay up to date with news and developments related to government publications by signing up for email alerts:
The Libraries of the House of Commons and the House of Lords provide research, analysis and information services for MPs, members of the House of Lords and their staff. Research Briefings are a useful - and independent - summary of information related to legislation: http://researchbriefings.parliament.uk/