WASHINGTON/LONDON (Reuters) -U.S. and British naval forces shot down 21 drones and missiles fired by Yemen-based Houthis on Tuesday towards the southern Red Sea, the United States said, with Britain hinting at further measures to protect international shipping lanes. British Defence Secretary Grant Shapps said it was the largest attack in the area by the militants to date as the three-month-long war between Israel and Hamas in Gaza spills over into other parts of the Middle East. “This is an unsustainable situation,” Shapps told reporters, adding “watch this space” with regards to further poss…