I mean in the most reductive, mathematical sense it will be sort of slightly true that if you're in a seat where Labour can win in the next election and you choose to vote for a different party then you will contribute to Tories getting elected instead. But if you were to vote for say the Greens, or even the Communists or whatever the left wing protest parties call themselves these days you haven't actually wasted your vote. You've effectively voted against the current political system and to shift the Overton window left.
A vote for the Greens will also result in their receiving more party funding from the state as they will likely exceed the threshold for where that applies. A Green vote also inherently signals strength of feeling on the climate which is itself critical to our survival. The climate isn't actually a "left" issue, whatever Murdoch and co want to peddle to you, it's an "I don't want my children to burn to death" issue. Think of it as voting for war, plenty of hard left and hard right governments and everything in between have gone to war.
And in terms of protest actual "hard left" parties, if you want to know what value they have, just look at the protests in France about the retirement age and how the "France Unbowed" MPs have got the brutal French riot police running scared because they're actually able to stick up for the protesters in parliament and give balance to the debate. There is no voice of protesters in Parliament in this country, just various shapes and sizes of apologist for the police, for repression, for the erosion of human rights. Just because the "hard left" can only get elected in this country when the Labour party establishment aren't looking, and even then they're under the thumb of the party whips to a greater or lesser degree, it doesn't mean that's the only way it can be. With PR where the actual spectrum of opinion is represented in Parliament, that's what functioning democracy looks like. Yes, including the fascists, the Communists, the centrists, everybody that you love, hate or are utterly indifferent to.
On the one hand, it's imperative the Tories don't win the next election because they will continue to destroy our democracy and make the country ever poorer and more extremist. But on the other, the restoration of our democracy by Labour by current appearance will be minimal. If you don't want to vote for that does it make you complicit with the Tories? Of course not, both are valid points of view and for both views it's important they are represented.
One thing that even the most narrow minded Starmer fans have to admit, each time the Tories get elected it gets ever clearer how incompetent, cruel, corrupt and malicious they are. You might think with Boris and Truss they hit rock bottom and now's the time to strike, but trust me it can get worse. They're still corrupt, incompetent and evil and that ain't gonna change just because Rishi knows how to fill a piggy bank. He himself isn't all that bright and he's clearly the pick of the bunch.