In 2012-13, the Queen Elizabeth II Conference Centre charged public sector clients (including central Government, the NHS and local government) a total of £540,562, excluding VAT, for room hire and conference facilities. Such usage reflects the limited availability of high quality venues in Westminster which can take conferences or gatherings with a large number of attendees, and significant clients include the Foreign and Commonwealth Office, the Department for Culture, Media and Sport and the Parliamentary Estates Directorate.
Such public sector spending has fallen in recent years, due to stricter spending controls in central Government. For example, the comparative figures for the last Administration were £1.8 million in 2007-08, £1.7 million in 2008-09 and £1.7 million in 2009-10. In 2012-13, Government bookings accounted for just 5% of its revenue.
I would note that the Queen Elizabeth II Conference Centre is a trading fund. Part of the trading fund surplus is returned to the Exchequer, and it contributed £2.3 million to the Exchequer in 2012-13.