UK Stocks-Factors to watch on April 19

April 19 (Reuters) - Britain's FTSE 100 index is seen opening 6 points higher at 7,323 on Thursday, according to financial bookmakers. * BHP BILLITON: BHP Billiton cut its 2018 fiscal year iron ore output guidance on Thursday citing issues in its railroad car unloading system, while also slightly raising its copper output expectations given higher production at the Escondida mine. * ASTRAZENECA: U.S. regulators have expanded use of AstraZeneca's lung cancer drug Tagrisso to incl