DTI
Index of UK public holidays.
http://www.dti.gov.uk/er/bankhol.htm

Lonely Planet
Information on the best time to visit Britain, major events throughout the year and public holidays.
http://www.lonelyplanet.com/destinations/europe/london/facts.htm#togo

Visit Britain
Important dates in the British calendar.
http://www.visitbritain.com/in/fact_files/planning/important_dates.htm