Here are the things I desperately miss, and sometimes pay hugely inflated prices for if/when I find them, and sometimes just flat-out cannot find:
Dutch-process cocoa
Baker's chocolate
Hershey's Kisses (yes, they're available here in a few places, for £3.50 for like five kisses. I can get them online cheaper but it's still quite expensive)
Nestle or any other semisweet chocolate chips. Also, any other kinds of chips, like peanut butter or butterscotch or those toffee chips with chocolate, because those don't exist here; neither does Semi-sweet chocolate and completely unsweetened baker's chocolate.
Canned pumpkin
corn syrup
cake flour (the only cake flour available here is self-rising; it's not a huge deal [the amount of leavener is pretty negligible so I just treat it like proper cake flour and leaven it myself], but I'd kill for some Swan's Down)
King Arthur or Gold Medal flour
grape jelly
Jell-o instant pudding mix (You can get Betty Crocker cake mixes at most stores here, btw, but no pudding mix at ALL)
Nutter Butters
Tootsie Roll Pops
Peanut Butter Oreos
Hamburger Helper (yeah, I know, but every once in a while it's nice to have something so easy and fast)
Kraft Macaroni & Cheese, both the kind with the powder and the fancier ones, like the four-cheese kind or the kinds with the melty cheese packet
Golden Grahams cereal (for Smores bars)
big soft marshmallows (it's VERY hard to get proper marshmallows; I have to buy six or seven little packages of mixed white & raspberry marshmallows and then pick the pink ones out)
Pringle's Light/low-fat (I used to find these sometimes. Not anymore. There are NO low-fat versions of Pringle's anywhere. So I don't eat Pringle's anymore because I'm not crazy about the heaviness of the original version)
Frito-Lay Corn chips!!! (I paid £5 for a tiny single-serving packet last year from Amazon)
Watermelon Jolly Ranchers
Iced Tea mix (especially Lipton peach; they sell individual bottles of it in a few places but they've changed the formulation and it tastes weird now)
Lay's Potato chips
Any condensed soups other than Cream of Chicken, Cream of Mushroom, or Cream of Tomato. Those are the only condensed soups you can buy here; no asparagus, golden chicken, celery, etc. Just chicken, mushroom, or tomato.
Arm & Hammer baking soda. (You can of course get baking soda here, in a little tub, as opposed to a nice big box of A&H for a dollar. Plus it just isn't the same.)
Kosher salt may be available in some places in the UK but not where I am. Iodized salt is very hard to find, and fairly pricey for a small tub. Mueller's egg noodles = expensive and really hard to find, too, so some medium dry egg noodles wouldn't go amiss if she likes making things like stroganoffs or coq au vin or any sort of Jewish dishes or...well, any of the thousands of things that are delicious with egg noodles (they sell "egg noodles" here but they're just pasta, they don't have the particular flavor of egg noodles as I know them). (Any Jewish foods are very difficult to find here; again, apparently that's not the case everywhere but it sure is in this area.)
SALTINES! And GRAHAM CRACKERS!!! Oh god...
Crisco shortening
Any cinnamon-flavored gum/candy/Altoids.
And yes, real Orville Redenbacher popcorn. I can get popcorn here at a few stores, and it is pretty good (I buy the kind that says "American Popping Corn") but it comes in a plastic bag, not a sealable jar (I do put it into my own jar), and it's just not Orville's. Also, butter salt isn't available here except a weird brand on Amazon, which I use but again, not the same.
I know there's more, but any or all of that would be a start.
