Look up some of the history of marks and Spencer's (how it all started) it is a customer service based role so you need to show you will basically go above and beyond for the customer. There will be a role play involved where you must show case this.
Be yourself and stay relaxed
To be your natural self and sell yourself and make shore you are spotlessly clean and tidy and speak when your spoken for
Try not to gouge the persons eyes out when all there doing is trying to look intelligent
Make sure you take your birth certificate and passport with you
First you should do a bit of a background check on the company to make sure you have plenty to talk about. I went for a more relaxed and jokey interview to show my personality. Ofcourse this will vary on the person you are.
Allways remember to smile
Business like clothes
Research the company but not a ridiculous amount, make notes and by all means ask questions