Smart casual! Dark jeans, shirt, simple shoes to compliment the outfit! Take this from a ex-supersales adviser that worked in Next! Creative agencies don't want to see you all serious and kitted out in a suit, they want to see you stripped back to basics, with an element of flare. Turning up in your scruffs is hardly advisable as it shows your lack of commitment to even looking presentable.
I went to my interviews in what I said above, when I got there they were all stood about in shirts and jeans. Now I go to work in whatever I pull out the wardrobe.....good times....