By Michael Holden and Kanishka Singh LONDON (Reuters) -WikiLeaks’ founder Julian Assange has appealed to the High Court in London to block his extradition to the United States to face criminal charges, his brother said on Friday, the latest step in his legal battle that has dragged on for more than a decade. Assange, 50, is wanted by U.S. authorities on 18 counts, including a spying charge, relating to WikiLeaks’ release of vast troves of confidential U.S. military records and diplomatic cables which Washington said had put lives in danger. Last month, Home Secretary Priti Patel approved his e…