Particularly hard to argue that 1955, 1959 and 1966 were not positive endorsements of incumbent governments. And I’d give the title to 1983, 1987 and 2001 as well for sure; 1950, 1992, 2005 and 2015 a bit more complicated as is 2019 (a rejection of the opposition).

Alternatively, you could argue that 1945 wasn't a positive endorsement but a negative judgement on the Party of Appeasement. Moving the baseline around is just definitional - it doesn't actually change anything.