As we all know, Stormtrooper helmets are neither works of artistic craftsmanship nor are they sculptures, therefore are not protected by copyright (see here). Their presence backstage at the Preen's London Fashion Week show is therefore not something that Disney, owner of the Star Wars franchise, is likely to be able to do anything about.
|Rodarte on the left; Preen on the right|
Preen took the Star Wars theme a step further however, as their collection includes a series of dresses featuring Darth Vader. Similarly, at New York Fashion Week, Rodarte's collection included silk dresses featuring Luke Skywalker and C3P0 prints. Preen and Rodarte may be licensed by Disney, however if they are not they could be in hot water: Rihanna famously succeeded in stopping Topshop from selling t-shirts with her image on them by claiming passing off (see here), however if Disney own copyright in the Darth Vader, Luke Skywalker and other images its claim could be more straightforward.