The influential British Prime Minister associated with the defeat of fascism in World War II is Winston Churchill. His leadership was pivotal during the Battle of Britain, where he rallied the British public with his stirring speeches, emphasizing resilience against the Nazi regime. Churchill's determination not only inspired the Allied forces but also reinforced the importance of democracy and freedom throughout Europe. His strategies and alliances, particularly with the United States, were crucial in eventually leading to victory in Europe, marking a significant turning point in the war history.