FTSE 100 dips, midcaps advance on Brexit day

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* FTSE 100 down 0.2%, FTSE 250 up 0.5%

* Britain to exit the EU at 2300 GMT

* Aston Martin roars to top of midcap bourse

* Boeing-supplier Senior hit by 737 MAX grounding

Jan 31 (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 surging 25% after Canadian billionaire Lawrence Stroll said he would take a stake in the luxury carmaker.

Senior Plc, a supplier to Boeing, underperformed the midcap index and slid 5.5% after warning revenue and margins at its aerospace division would decline in 2020 due to the 737 MAX jet grounding. (Reporting by Shashwat Awasthi in Bengaluru; Editing by Bernard Orr)