You have to look at it as the game and subsequent acts being part of the overall package.
At that KING tier they’re also giving you a copper engraving of some kind nobody below that gets. They will put your name (if I read it right) on a crucifix somewhere within the game itself. You also get the art book, the silver coin, the signed poster, shirt, dice, etc.
Should they have offered a tier that would cover JUST the three acts alone? You could make that argument. However, I’m pretty positive it won’t cost anywhere near this tier just to get the three acts themselves. That’s in large part due to the perks, which is a primary function of Kickstarter’s crowdfunding. It’s intended to give backers something significant in return, especially if they’re going to spend more on a higher tier.
Otherwise, you’d be better off going with a more basic backer level and get the later acts when they release as DLC or however they do it. Most DLC is no more than $15-20 USD anyhow (or 9-12 GBP), if that.
As it is, they’re selling the basic game for just 15 GBP, or $24 USD, as digital only with no other perks, slightly more if you want a different version. For people who don’t back anything here, I don’t know what the final retail price will end up being. It could be the generally typical 40 GBP or $60 USD for a full game or it could be less.
I don’t know if the additional acts will be treated like normal DLC or add-on expansions - if it was mentioned somewhere I already forgot the answer.
But yeah, in the end Kickstarter or Indiegogo operates on the same basic premise of giving backers who pay more a reward beyond the basic purpose of the project itself.
(Edit: ah, perfect - there’s the answer about it right above)