On 10th May 1940 Winston Churchill was appointed Prime Minister of the United Kingdom following the resignation of Neville Chamberlain. Churchill remained in power for the rest of World War II and returned as Prime Minister once again from 1951 to 1955. 

In the photograph below, taken almost exactly 5 years later, Churchill can be seen addressing crowds from the balcony of the Ministry of Health building in Whitehall, London on 8th May 1945, the day on which World War II officially ended in Europe (VE Day).
Source: Imperial War Museum H 41849 
To read Churchill's speech to the crowds, and to hear his announcement of the unconditional surrender of Nazi Germany follow this link.