"So you voting rational is entirely based on "I want to vote for Party B because I really want to get rid of Party A"
And not because Party B has any credible plans to improve things / keep them the same or potentially make them worse. It's just that they are not Party A."
That's not exactly what people are saying.
I'm not voting for Party A - because they have broken the social contract that presumes that if I pay my taxes and obey the laws of the country, the government will provide (or facilitate) things that I can't personally afford, like infrastructure, healthcare, education, policing and defense, and at all times will act in the best interests of all of the country and not just in the best interests of their sponsors.
This government has broken this contract by selling off the vital public services and allowing their chums to run off with the assets, or by underfunding these vital services to the point where people who can afford to pay are forced to go to the private providers where they are fleeced, and everyone else has to make do with sub-standard service or join the end of a very, very long waiting list. All this while they themselves break the law, lie and misrepresent the facts.
At the same time, they have tried to silence my opposition to their antics - by banning strikes and protests and attempting to define anyone who disagrees with them as "terrorists" or at the least as being "unpatriotic." I've been pejoritively labelled by one PM as a "Remoaner" for pointing out that Brexit would be, and has turned out to be, a catastrophe. I've been called a "Citizen of Nowhere" by another PM for daring to have lived abroad and to openly state that not everything that the UK does is "world-beating." I've been told to "fuck off back where you came from" by a man in a smart suit for daring to speak to my daughter and a friend in a foreign language on a train in England (I'm English, which then seemed to enrage him even more).
So no - I'm not voting for Party A. Which leads to the question - who do I vote for?
Not C, D or E because my vote would be wasted in a constituency that is really a two-party fight. So under FPTP I'm left to choose between Party B or abstaining. I don't want to abstain, particularly if that means that Party A sneaks in because not enough people voted for the only viable alternative. So despite some misgivings, it has to be Party B, which more closely aligns with my views than Party A, even if I'm not 100% in favour of everything that they propose (and I'm pragmatic enough to know that they won't be able to clear up the mess they inherit in just 5 years).