John Lewis is an upmarket department store selling clothing, accessories, cosmetics, electricals and home ware. As they appeal to a specific market sector, they look for staff that fit in to that. This means that they are not necessarily looking for particular skills, but personality, at least for the roles in branches anyway.
This means that you need to be yourself and make sure that your personality comes over. You need to prepare for the questions they ask though. These are usually very typical interview questions such as why do you want to work for the company, what skills can you bring, what experienced do you have, where do you see yourself in ten years. You may already have answers to these questions, but make sure that you practice them and orientate them towards John Lewis specifically.
You may get asked to sell items so you need to be prepared for that. You may also be put in groups and have to complete tasks. These can be hard to prepare for but make sure that you have something to add, even if you agree or disagree with someone else. Try to put forward some new ideas. You may have to present in front of all of the applicants so be prepared for that as well. Try to be confident and not too nervous if you can.
It will be useful to find out a bit about the company and their history. Perhaps find about the branch too and what they sell there. If you are applying or a specific department, even though they tend not to be so concerned with knowledge in that area, it would put you at an advantage if you had some. If they have two interviewees with similar personalities, then some extra knowledge could put you ahead. Therefore make sure that you at least know the basics.
Make sure that you are enthusiastic about the company and working for them and answer the questions. It might sound obvious, but with a group interview, like these tend to be, it can be easy to get nervous and waffle without really getting to the point. This is not the best way to sell yourself.