Which prominent figure served as the UK's Prime Minister for the longest duration during the 20th century?

Margaret Thatcher, often referred to as the "Iron Lady," held the office of Prime Minister from 1979 to 1990, a total of 11 years. This makes her the longest-serving British Prime Minister of the 20th century. Her tenure was marked by significant economic and social reforms, including privatization of state-owned industries and curbing the power of trade unions. While these policies were controversial, they undeniably shaped modern Britain. Tony Blair, who served from 1997 to 2007, also had a significant run, but it fell just short of Thatcher's. John Major, who succeeded Thatcher, had a shorter term. So, while Blair and Major were important figures, Thatcher's 11 years in power set her apart in terms of longevity during that century.