I've been in London for 2 years (from Canada) and I've found it takes quite a bit of searching, but a lot of the Canadian goodies I craved during the initial few months I have now managed to find here. (Or have found reasonable ways of ordering them online from companies within the UK.)
Something I really crave, and cannot find, is Coke slurpees. All summer long, I dream of them. The closest thing available is a slush cat, buyable at cinemas and at some small newsagents/shops here, which basically come in flavours of red or blue. They are very very sweet, and nothing like a coke slurpee except in consistency.
I like to cook, and I struggled for a few months to locate fine egg noodles. The typical ones here are sold in the Chinese section, usually Sharwoods brand, and smell terrible and have the wrong consistency (compared with the lovely Catelli ones from Canada). But more recently I've found an imported version from
Poland I believe, at the larger Sainsburys.
I also couldn't find canned pumpkin anywhere, and I do love my pumpkin pies and soups. But during the holiday season I did find this at Waitrose, and stocked up for the next year.
The flour here is not the same as Canadian all-purpose! I couldn't work out why my baking and sauces were not the right consistency until I explored this. "All purpose" doesn't exist here. "Plain flour" is NOT the same thing. If you cook/bake, your recipes may need re-working. I'm still experimenting, but so far "strong" flour has yielded the closest results for baking, and "sauce flour" for sauce. Also note Corn Flour seems to be much stronger than Cornstarch (which afaik doesn't exist here).
I haven't found molasses.
Cover girl makeup products are here, Always feminine products are here, Doritos are here. French's Mustard - I've found this at Sainsburys.
I have found Taco Bell seasoning packets, jars, taco kits here in the larger Sainsburys in London. They are expensive... but needed sometimes.
Tomato sauce - it doesn't exist here, but you can buy Tomato Puree (aka "tomato paste" in Canada) in cans or in tubes. Mix 1 part tomato puree + 1 part water = tomato sauce. Works great in all my recipes.
Things I still stock from Canada: proper deodorant! shampoo (or I order it in - I like a particular salon shampoo from Canada; note that salon-quality shampoos are so expensive here that it's cheaper to order Canadian ones online!), and most of the items others have mentioned in this thread.