What happened in the last election, for example, is that "small C" conservative voters had no one to vote for. The actual Conservative party was unelectable, mired by scandals and gross incompetence. Labour party, the other one of the two, was anathema. Taking smaller steps to the left or right, you have the Liberal party or Reform, neither of which had any chances of getting a significant number of seats in the parlament.
In a healthy multi-party system, some other "small c" Conservative party would spring up to hoover up the votes. As it is, people had the choice of voting for a party widely seen as incompetent (including by its own sympathisers), a party that wants the opposite of everything they like, or a fringe party with no chance of shaping UK politics.
I have some theories as to why these happen at the same time... But anyway, it's not a uniquely American phenomenon.