(Reuters) - London's FTSE 100 slipped lower on Friday as coronavirus fears intensified and as a firmer pound dragged exporter shares lower, while Aston Martin drove the midcaps higher, hours before Britain leaves the European Union.The main index gave up 0.2%, as exporters such as HSBC and Diageo fell. Risk sentiment was also hit after the World Health Organisation declared a global emergency over the spread of the newly-found virus.The FTSE 250 recorded a 0.5% gain, with Aston Martin ..