Read TFA -- Guardian decided to destroy the materials in order to protect its journalists. They were afraid that recovered computers would contain information about which Guardian staff accessed the machines and when, and that could be used as evidence against them.

The material was destroyed in this way because the alternative was to hand it all over to the UK government agency mentioned.

...what did the security services expect to achieve here?

To stop or severely impair The Guardian's ability to report classified materials.

Intimidate. Scare. Show their higher ups they were doing something. You know; the usual totalitarian M.O.

I think we are all aware of what they wanted to achieve, but some are afraid to say it.

The usual interpretation has been that UKgov was clueless about technology, refusing to recognize that there were other copies elsewhere. But maybe they knew that it was only "security theatre" and the intent was to only make threatening gestures without really affecting the situation.

It's still a threat of something worse in future, but in comparison with the way things have been going in western/northern countries recently, it was a pulled punch, a kind of ritual or "CYA" move.

My guess is that they did it to make their bosses happy

Scare Tactics: "House this data and we'll fuck with you"

They were enforcing the law. There is no 1st amendment protections in the U.K.