In this case, I think British law is saying that the items used to make Star Wars, such as the plastic suit on the wooden film set were not created initially to be valued in themselves - but that their purpose was only to represent fictional characters in a story. The film itself as a whole would be the work of art, and not the tools-of-the-trade that were used to make it. Once the film is completed, and their reason for being built is over (I think the law is saying) they have no artistic purpose left.
... if I've grasped this correctly then LFL have dropped the ball. They should have pressed home that no matter how British law defines their reason for being built originally, these days stormtrooper helmets have become works of art. They stand in glass cabinets in museums the world over for the purpose of being admired for what they are by people that appreciate the film. Props used to be thrown away when a film was finished, but these days they often take on a new life as objects in collections or displays.
In this case there's no difference in purpose between a stormtrooper helmet on display in a museum, and a piece of sculpture that people who like these things pay money just to go and see. It doesn't matter why either of them was made in the first place. A shark in a tank of formaldahide is accepted as a work of art, but the thing was born as a fish!