UK Stocks-Factors to watch on April 20

April 20 (Reuters) - Britain's FTSE 100 index is seen opening down 20 points, or 0.3 percent lower on Wednesday, according to financial bookmakers. For more on the factors affecting European stocks, please click on * The UK blue chip index closed up 51.83 points higher on Tuesday at 6,405.35, having risen to its highest level in four months, lifted by stronger mining and banking stocks. * BHP BILLITON: Mining giant BHP Billiton followed rival Rio Tinto in trimming its iron ore o