I had an interview for Topshop/Topman which they called a 'Recruitment Event' and what they said to you sounds pretty similar to mine, and I also have a friend who went for an interview at River island and got the job. Don't wear office/formal clothes. At my interview, a girl turned up in a suit and heels and just looked very silly and out of place, as did a guy in a suit - you are dressing how you would if you actually worked there, so wear all River Island! Obviously don't look too scruffy though, just dress like the shop assistants do when you go in there. Basically, they want you to sell the brand with your dress sense.
You will probably have a group interview and do some games like perhaps dressing a mannequin for a certain occassion or producing a poster about customer service, just working in groups. I'd suggest you practice and think of loads of ways to provide good customer service and how to sell the brand, because they will really hammer that home to you (even though their customer service is terrible in my local river island...) Remember to speak up, DON'T INTERRUPT PEOPLE and be really really bubbly.
You'll then probably have a 1-2-1 interview, where they will again ask you about customer service, team playing and describe a situation and ask you how you'll deal with it.
If you're a confident and bubbly person then you should have no problem! good luck