I went to Whole Foods right after arriving here last November -- it was Thanksgiving Day, we'd moved to London less than a week earlier and I was feeling all out of sorts and desperate for something familiar.
The spirit and atmosphere of the store felt very familiar, but it's definitely not chock full of US-specific stuff. You'll be disappointed if that's what you're expecting. As everyone else has said, it's massive and wonderful and expensive (that felt familiar, too!) and worth a trip just for the food p0rn.
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I did leave with turkey, potatoes, green beans, cranberry sauce and pumpkin bread for what we now refer to as our most expensive turkey dinner ever (per ounce)! At the time, it was well worth it for the cheer-up.