That someone is Natural England, who is tasked, by law, with enforcing laws that protect wildlife and the environment and needs to sign off on disruptive work:
> A spokesperson for HS2 Ltd said "multiple options" had been considered, including green bridges and restoring habitats, to "comply with laws protecting vulnerable species".
> It said through "extensive engagement" with Natural England, "a covered structure was designed".
* https://www.bbc.com/news/articles/c9wryxyljglo
If you don't like it change the law so that the environment/wildlife isn't protected, or these kinds of sign offs are not requirement, or can be overrided in the enacting legislation of infrastructure projects.