Got to remember once i get a job there (if i do), if there is a Waitrose at my uni (if i get in), i can transfer my job, which can only be useful.
It's worth it even if it's just for the experience. Many of the bigger employers use similar group exercises in their interview processes, so it's worth getting the practice in now, so that once you've got through uni and want to move onto a good career, you've had a bit of preparation.
I would suggest thinking about internships / work experience relating to your career of choice during the longer holidays while at uni, but having Waitrose on the side will do you no harm to say the least.
[EDIT: And I've just realised I haven't really added to your initial question. I can't comment on the exact kind of thing you'll be facing, but if it's a group exercise then bear in mind some of the obvious things - get involved, and speak up, but make sure you don't trample the other people there to get yourself heard. Attempting to bring other, quieter people into the discussion (if it's that kind of thing) always wins brownie points, and if it's a timed task, keep an eye on the time so you can keep your group focussed. So often in these exercises no one thinks of that, and all of a sudden time runs out...
In terms of how to present yourself, you can't go wrong with a suit and tie (but nothing too bright / crazy design) if you own them.]