Dress appropriate and look smart and always have a C.V. and Cover Letter about your work experience history at all times that's should be the way to prepare for an interview all the time.
Read up about the origins of the company, possibly visit one of the stores to get a vibe for the place and look at the type of products Smiggle sell.
You need to comeaccross as a person who will stir things up every chance you get
The interview will consist of group activities such as introducing yourself to the group, selling one of their products to the group and a team work related activity to show how well you work with others.
Work out your availability and flexibility as that is their main concern. don't really have to dress up for the interview, just don't go to it looking a scruff
Be funny, don't hold back your humour
Be yourself, they love to see your true colours
Have fun with the tasks, I loved it