> How is this different than refusing to tell where you've buried the bodies?

It would be more equivalent to refusing to let the police search one room in your house after they’ve established a reason to believe that you might be hiding bodies in it.

> Why should the police be permitted to assume you're guilty and in possession of vital information under certain circumstances?

They haven’t assumed guilt of the suspected crimes. However, if they have sufficient reason to believe that evidence of a crime exists in a certain location then they can compel someone to provide access to it. It’s similar to how the police can search your private property if they have sufficient evidence that a crime has been committed

That said, I don’t know the standard of evidence necessary in the UK to get this far.