"I don't like any of the parties." then form one of your own.
"None of them have a manifesto where I solidly support the majority of their points. Or even many of their points." Then join a political party and campaign for what you want or write to your MP to influence them.
"None of them are going to/going to be able to fulfill all their promises anyway." Possibly neither is anyone else you meet, everyone if flawed, even the M and S food advert is an exaggeration, you know, but we still love food!
"Most of them are inconsistent and hypocritical." Maybe, yes, but again, so are a lot of people but we need to engage with them, hold them to account.
"From what I understand, protest votes get counted so if they're enough of them it would make a valid point."
What is the point of making a point? Who do you think will step in to run the country because of a protest vote?
"Surely no one can say that they 100% agree with one party totally and 100% reject the others?" That is not really it though, is it.
You are voting for the party, or leader, or party and leader that you think will do their best to fulfill the policies that best represent their party and that you most agree with.
If you really cannot do that then it doesn't much matter if you spoil your paper or just don't vote, IMHO. But there is nothing to stop you from putting yourself in the picture if you think you could do better.