UK Stocks-Factors to watch on May 24

May 24 (Reuters) - Britain's FTSE 100 index is seen opening down 20 points, or 0.3 percent lower, on Tuesday, according to financial bookmakers. For more on the factors affecting European stocks, please click on * The UK blue chip index closed 0.3 percent lower at 6,136.43 points on Monday, with a drop in the prices of crude oil and industrial metals putting pressure on commodities-related stocks. * SHELL: Dutch pension fund PGGM, a major shareholder in Royal Dutch Shell , critic