By David Milliken and Andy Bruce LONDON (Reuters) – The Bank of England raised interest rates by the most since 1989 on Thursday but warned investors that the risk of Britain’s longest recession in at least a century means borrowing costs are likely to rise less than they expect. The BoE increased Bank Rate to 3% from 2.25% and warned that the British economy might not grow for another two years – the longest slump in records dating back to the 1920s – if rates were to go up by as much as markets have recently bet. “We can’t make promises about future interest rates but based on where we stand…