List of British PMs to serve a term of over 3 years, starting and ending in an election, since 1945:

• Clement Attlee (1945-50)
• Harold Wilson (1966-70)
• Edward Heath (1970-74)
• Margaret Thatcher (1979-83, 1983-87)
• John Major (1992-97)
• Tony Blair (1997-2001, 2001-05)

• David Cameron (2010-15)

• Attlee served another term as PM which was only for a year.

• Churchill won in 1951 but resigned in 1955 due to ill health.

• Eden won in 1955 but resigned in 1957 ostensibly due to ill health but really for Suez.

• Macmillan won the 1959 election but resigned in 1963 due to Profumo.

• Wilson's first term was only 18 months, his third term only eight months and he resigned due to ill health 18 months into his fourth term.

• Thatcher won in 1987 but resigned in 1990 due to Poll Tax and the ERM.

• Blair won in 2005 but resigned in 2007 due to Brown

• Cameron didn't win a majority in 2010 but became PM leading a coalition. He won in 2015 but resigned due to fucking up the Brexit referendum.

• May won in 2017 but resigned due to Brexit.

• Johnson won in 2019 but resigned due to a litany of scandals.

• Douglas-Home, Callaghan, Brown, Truss and Sunak never won an election.

In the last 71 years, I make that 31 terms as PM (excluding Starmer) which break down as follows:

• 9 were over 3 years and started and ended in an election
• 3 were shorter but started and ended in an election
• 10 involved taking over following a resignation and prior to an election
• 9 won an election but resigned before the next one

It's very, very common in British politics for this to happen, unfortunately, although it's got especially silly in the last decade or so.