WASHINGTON/BRUSSELS (Reuters) – The United States is not keen on pursuing a separate free trade deal with Britain if it leaves the European Union, U.S. Trade Representative Michael Froman said on Wednesday, the first public comments from a senior U.S. official on the matter. Froman’s comments undermine a key economic argument deployed by proponents of a British exit, who say the United Kingdom would prosper on its own and be able to secure bilateral free trade agreements (FTAs) with trading partners.