Your question reminded me of this passage from Profiles in Courage by JFK, and depending on context, I would say the person is "courageous".
It takes great courage to do what you think is right even though it may mean the end of your career and the dislike and criticism of your friends and neighbors. Many people never have the opportunity to show such courage. But all of us have the opportunity to recognize such courage in others, to respect the person who is doing what he believes to be right even though we think he is wrong......Courage is much more than bravery on a battlefield; it can mean acting according to your beliefs whatever the consequences......We can all share in such courage by refusing to join with those people who make unreasoning attacks on the man who is doing or saying what he honestly believes to be right.
John Fitzgerald Kennedy, Profiles in Courage, Harper & Brothers, New York, 1955.
I think that adjective fits Baroness Thatcher perfectly.