Winston Churchill

Formal switch, twice. Churchill crossed the floor from the Conservatives to the Liberals in 1904 over free trade, serving in Asquith's reforming cabinet, then drifted back to the Conservatives in the 1920s and became Baldwin's chancellor. He later joked that 'anyone can rat, but it takes a certain ingenuity to re-rat'. Both switches were driven by policy, tariffs and anti-socialism, and both advanced his career, which is unusual.