Hi there, I used to work at Waitrose.
If you've got the interview you're already at a great place. They usually have quite a lot of jobs going at any one time, and as I recall I was in an interview with 3 or 4 others, and I'm sure I saw at least 2 of them while working, possibly they all got the job.
What I will say from my experience there, is that it's all about presentation. Do not kid yourself and think that you can't do the job. ANYBODY can do the job. It's not hard to stack shelves, or scan items. There'll be the usual questions about teamwork etc (if you have never been in an interview look up common interview questions). Just remember that to have made it to the interview (and got past the rather tricky Waitrose online test), you must have half a brain, and they believe you are capable of doing the basic and menial tasks they will ask of you.
What they want to see is how well you present yourself, how polite you are, whether you look like you're not going to come in on a Sunday morning because you were out drinking all night, that kind of thing.
The questions that they ask you are not going to be of great importance. Every employer asks them, and honestly I don't think they give a crap about how you have proven skills of working in a team because you played football at school and it was important to work together to win. No matter the person, you will have a personal example to support each question that they ask you. What you need to do is just try not to crumble under the pressure. If you do find your mind goes blank, just say so, and try and kinda joke it off, they know interviews are hard and stressful, and see this kinda thing all the time. The people who work in Waitrose are generally very nice, they're not trying to trip you up.
Dress well to the interview. This is important. Don't look like a mess. - I will say, I am a bit of a mess. I never look that good, my hairs always messy, I arrived to the interview on a 100cc motorbike and I think it was raining recently, so I wouldn't have been that clean. I was wearing a suit, and I'd shaven and showered before hand. I tried to look and sound confident and correct, and just generally gave the vibe that I'd be a good candidate.
The questions aren't what matters here, it's just the way you carry yourself. You have to look and act as though you're ready to represent the supermarket that caters to the middle and upper class in society. Try and look as though you're from that world, and you'll fit right in.
They're probably more relaxed than I've described, but this is what they're looking for. You'll be fine, don't worry about it too much, really it's not hard to get a job there. I got a job there, and I'm a bit of a hippy scruff. My sister got a job there, loads of my friends had jobs there while they were in school etc.
It's not hard, just look presentable and answer the questions honestly and to the best of your ability. Otherwise chill out, relax, and don't worry about it