(Reuters) - Britain's main index fell on Tuesday, weighed down by disappointing results from blue-chip heavyweight HSBC and a stronger pound, while mid-cap baker Greggs surged to a record high after guiding to higher full-year profits.The FTSE 100 closed down 0.6 percent, its biggest fall in nearly two weeks, and the FTSE 250 was down 0.3 percent.HSBC was the biggest drag on the main bourse on its worst day in almost two years, as uncertainty around the China-U.S. trade situation was reflected..