@mavu @purplelotus13 @pluralistic
Well, kind of. They don't influence the election outcome, but they do influence the policies of the people who want to win next time. The rise of UKIP (and now, Reform Ltd) is a big part of why both of the major parties moved to the right.
Voting for a far (or, at least, further) left party may not get them into power, but it sends a signal the leftmost major party that they've losing votes to the left and need to swing back there or they'll lose elections in the future.