Your experience working at Waitrose will mainly boil down to how competent and strict your manager is. I believe I mostly got lucky as I was able mostly at my own pace and could take my breaks when I wanted. This will likely not be the case for too many people.
The job itself is ok, although it's extremely easy to hit a wall for development after 6 months and advancement is on your own initiative. Running for things like forum rep and being a hardworker will put you in good stead, although I personally know employees who have worked for 5,10,15 years in the same position and haven't been allowed to advance.
Worst thing about the job are rude customers and the insanely monotonous work you do for 38 hours a week. "Facing-up" shelves for 7-14 hours a week, stacking shelves and occasionally being able to help out a customer.
Listening to music/radio/podcasts before the store opens was the only saving grace from the work.