LONDON, July 21 (Reuters) - Britain’s government will have to be nimbler in the way it manages the public finances if interest rates go up because government bonds carry shorter maturities than in the past, the head of the country’s budget watchdog said on Wednesday. “It just needs to be more alive to factors that might drive interest rates higher, and in effect what it means is that it needs to be more nimble with primary spending or non-interest spending if it wants to be able to respond to a sudden rise in interest rates,” Richard Hughes, chairman of the Office for Budget Responsibility, said. Reporting by David Milliken, writing by William Schomberg Our Standards: The Thomson Reuters Trust Principles.