UK Stocks-Factors to watch on Dec 13

Britain's FTSE 100 index is seen opening 1 point higher at 6,881 on Thursday, according to financial bookmakers. * SAINSBURY: Sainsbury's and Asda, the British supermarket groups that want to combine their businesses, will challenge a refusal by the regulator probing their 7.3 billion pound deal to give them longer to respond to the latest evidence. * VODAFONE: Australia's competition regulator on Thursday expressed preliminary competition concerns about the proposed merger between TPG T