Time for Britain to get used to seeing more red. Walgreen Co., the U.S.’s largest drugstore chain, announced Tuesday that it will pay $6.7 billion for a 45 percent ownership stake in the British drugstore chain Alliance Boots. Walgreen will have the option to buy the remaining 55 percent within three years. The company will pay $4 billion in cash and the remainder in stock, according to a statement. Alliance Boots was bought in 2007 by KKR & Co. and chairman Stefano Pessina in a £12.1 billion bailout.