Presumably that has nothing to do with this particular article, which in all likeliness was written by some intern and not the big deal we think it is. Whatever their general reasons are to resist FOIA requests, they probably just apply those across the board in order to not set precedent.

I don't know enough about UK law and journalistic process to know why they resist it, but in the general case I could imagine it's related to protecting sources. If you're handing over a pile of docs in a hurry and forget to black out a single cc, you could get a source compromised.