I can't fathom why a left wing voter would choose a very right wing option.
I think this sentence sums up why you can't grasp the arguments people are making on this thread.
You class yourself as left wing. That is your tribe and you can't imagine voting for a right wing party.
Because you have aligned yourself with the tribe of left-wingers, you assume everyone else also aligns themselves with a political tribe, and always vote according to that alignment.
Many people don't think of themselves as left wing or right wing. They vote for a candidate or party which most closely represents their own views at the time of the election. Or they are persuaded by some aspect of what a particular party is promising to to.
If you think about it, there must be many such people, otherwise we would always have the same party in power, yet there can be huge swings in voting towards left or right compared to the previous election.
You say that you can't imagine why someone who voted Labour in 2019 might vote Tory or Reform in 2024. Presumably that means you also can't imagine why someone who voted Tory in 2019 would vote Labour in 2024, yet if the polls are correct, then there must be many such people.
It's the floating voters without the tribal party alliances who decide the outcome of the election, not the party faithful who always vote the same way.
And just a final comment on this:
other left wing options are available
The left wing parties are all in thrall to gender ideology. If you're gender critical there is no left wing option.