UK Stocks-Factors to watch on April 28

April 28 (Reuters) - Britain's FTSE 100 index is seen opening down by 23 points, or 0.4 percent, on Thursday, according to financial bookmakers. For more on the factors affecting European stocks, please click on * The UK blue chip index closed up 35.39 points, or 0.6 percent, at 6,319.91 on Wednesday, boosted by strong results from drugmaker GlaxoSmithKline, while banking stocks were mixed as traders digested latest figures from Barclays and Standard Chartered. * TUI: European to