UK Stocks-Factors to watch on May 18

May 18 (Reuters) - Britain's FTSE 100 index is seen opening down by 20 points, or 0.3 percent, on Wednesday, according to financial bookmakers. For more on the factors affecting European stocks, please click on * The UK blue chip index closed up 0.3 percent at 6,167.77 on Tuesday, with house builders helping the index higher after Taylor Wimpey announced a special dividend payout to lead gains in the sector. * SHELL: Two investor advisory firms have recommended Royal Dutch Shell