If a party doesn't get an overall majority in a British general election, it hasn't "won" that election, end of.
Of course, by practising the black arts of politics and performing various manoeuvres and shenanigans...effectively, greasing someone else's palm in one way or another...a party leader MAY wheedle his way into power, but that's not "winning".
If a football team wins a match by scoring the only goal of the game as a result of a penalty gained by diving, they haven't "won" the match in any true meaning of that word, they've stolen it!