UK Stocks-Factors to watch on May 31

May 31 (Reuters) - Britain's FTSE 100 index is to open 4 points higher at 7,693 on Thursday, according to financial bookmakers. * WPP PLC: Proxy adviser Institutional Shareholder Services (ISS) has offered its support to WPP Plc's executive chairman, Roberto Quarta, by recommending that shareholders of the world's largest advertiser re-elect him at WPP's annual meeting on June 13. * HSBC/Royal Bank of Scotland: HSBC Holdings Plc has interviewed outgoing Royal Bank of Scotland C