LONDON (Reuters) - Asda, the British supermarket arm of U.S. retail giant Walmart , reported lower sales in its latest quarter, saying uncertainty surrounding Britain's exit from the European Union had negatively affected spending patterns.The group, whose attempt to be taken over by rival Sainsbury's for 7.3 billion pounds ($9.3 billion) was blocked by Britain's regulator in April, said on Thursday comparable sales fell 0.5% in its fiscal third quarter. That compared with a second quarter rise..