UK Stocks-Factors to watch on May 18

May 18 (Reuters) - Britain's FTSE 100 index is seen opening 51 points lower on Thursday, according to financial bookmakers. * BHP/ELLIOT: Activist investor Elliott Management on Thursday said a meeting with BHP Billiton Chief Executive Andrew Mackenzie in Barcelona on Wednesday had been "constructive". * BP: BP Plc's first foray into Mexico's recently opened energy market is proving more promising than expected, and the government should offer more big projects to lure investmen