By Sachin Ravikumar and Elizabeth Piper LONDON (Reuters) -British Prime Minister Liz Truss warned of tough times ahead after she scrapped her vast tax-cutting plan and said she would carry on to try to put the economy on a stronger footing, defying calls for her resignation. After weeks of blaming “global headwinds” for investors dumping the pound and government bonds, Truss on Monday said she was sorry for going “too far and too fast” with her radical economic plan to snap Britain out of years of tepid growth. It was not clear whether the apology would quell a growing rebellion in her Conserv…