Can you think of anything?
It doesn't seem extraordinarily expensive given the cost of building anything these days, I'd question should the cost of building new things be so expensive, rather than should money be spent on this kind of project, because of all things to spend a large unit of money on, this does seem like a useful one.
You keep making the assumption that the building has to be built at all, and that 200 million isn't unreasonable. It does not have to be and its for _bats_.
but you don't need to build it at all
you could fund a lot of bat reserves in perpetuity with even 10% of that money
instead of a 1km long concrete box
Ya but funding bat reserves has nothing to do with a long concrete box, unless it also literally is a bat reserve. The money for the bats can come from the bat fund, and the concrete box should be able to come from the concrete box fund, if there's not enough for both, figure out which one is more impactful for the people paying the taxes and persuade them to let you save the bats, or let them do it through personal acts of charity.
Yes, lots! £100m could give a hospital a substantial renovation for example.