For information on Parliament see:
http://www.parliament.uk/
For information on the UK Government see:
http://www.direct.gov.uk/
For information from the UK Prime Minister’s Office see:
http://www.number10.gov.uk/
For information on HM Courts’ Service is:
http://www.hmcourts-service.gov.uk/
The UK Supreme Court website is available at:
http://www.supremecourt.uk/
For details of the Judicial Committee of the Privy Council, see:
https://www.jcpc.uk/
For information on the monarch and the monarchy, see:
http://www.royal.gov.uk/
Many of the constitutional institutions may be followed on Twitter, too:
Parliament:
http://twitter.com/ukparliament
Prime Minister's Office:
http://twitter.com/DowningStreet
Monarchy:
http://twitter.com/BritishMonarchy