In February 1975, Margaret Thatcher was elected leader of the British Conservative Party. The Institute of Economic Affairs arranged a meeting between Hayek and Thatcher in London soon after. During Thatcher's only visit to the Conservative Research Department in the summer of 1975, a speaker had prepared a paper on why the "middle way" was the pragmatic path the Conservative Party should take, avoiding the extremes of left and right. Before he had finished, Thatcher "reached into her briefcase and took out a book. It was Hayek's The Constitution of Liberty. Interrupting our pragmatist, she held the book up for all of us to see. 'This', she said sternly, 'is what we believe', and banged Hayek down on the table".

What political party was Margaret Thatcher the head of in 1975?