April 20 (Reuters) - Britain's FTSE 100 index is seen opening 21 points higher at 7,350 on Friday, according to financial bookmakers. * SHIRE-TAKEDA PHARMA: Rare disease drugmaker Shire Plc said on Thursday it had rejected a $63 billion cash-and-stock acquisition offer by Japan's Takeda Pharmaceutical Co Ltd, while Allergan Plc reversed course on pursuing a rival bid. * RANDGOLD RESOURCES: Work has resumed at two gold mines operated by Randgold Resources in Mali, after strik