(Reuters) - UK shares tumbled on Thursday as the rising death toll from the coronavirus outbreak led investors away from risky assets like stocks and at home the Bank of England kept interest rates unchanged, while Shell and BT slid on disappointing earnings.The main index fell by 1.4% to levels not seen since mid-December and the FTSE 250 slid 0.9%, as a modest rebound that the indexes had managed over the last two sessions unravelled.Countries have begun isolating hundreds of citizens..