Usually smoking and drinking in the House of Commons chamber is against Parliamentary rules but due to tradition the Chancellor when giving the budget is allowed to choose his drink. In this case George Osborne chose alcohol when previously he had stuck to water.
Dates for when the practice became tradition are sketchy but Benjamin Disreali when he was Chancellor in 1866 was reputed to have drank alcohol too.
Parliament.uk source under 'Despatch box drinks'