By David Milliken LONDON (Reuters) -Britain faces an unsustainable debt burden more than three times its current level unless future governments raise taxes to fund increasing costs from an ageing population and falling revenue from taxation on motor fuel, a watchdog warned on Thursday. Britain’s Office for Budget Responsibility (OBR), a government agency, said debt was on course to reach almost 320% of gross domestic product (GDP) in 50 years’ time – up from 96% currently – if successive governments did not tighten fiscal policy. “The pressures of an ageing population on spending and the loss…