I moved to London from Los Angeles, and was afraid I'd have a hard time finding the specialty…read moreingredients I was used to finding so easily in California. But I shouldn't have worried, because United Supermarket is a treasure trove of specialty ingredients, for a wide range of Asian cuisines -- Filipino, Vietnamese, Japanese, Thai, Chinese, and more. There's also prepared sushi and bubble tea, which I haven't tried.
You can find any sort of sauce or vinegar you might want (fish sauce, soy sauce, cooking wine, etc.), as well as a wide range of cheap bagged spices. There's canned green jackfruit and coconut milk, and even canned hummus and baba ghanoush. There are lots of prepared frozen foods, like dumplings, spring rolls, etc. You'll find any sort of organ meat you might want, along with a lot of frozen and fresh fish -- they even had black silky chickens available. Explore the aisles and see what inspires you!
If you're not as adventurous a cook, United Supermarket is still well worth checking out for the cheap produce. But be warned; cheap produce like this spoils quickly, and check the expiration dates. I bought mushrooms that I noticed were already a day past the sell buy date, and that went bad in a couple of days sitting in my fridge.
They have cool grocery baskets on wheels that you can lug around the store, but you're not allowed to take them outside. So if you're planning to buy more than you can easily carry to your car, pony up a refundable $1 (yes, $1, not 25 cents!) to get a cart.
They also cut keys for $2. I had them cut a key for me, and it didn't work (it was cut on a key that was slightly the wrong size.) I returned it a few days later, and they cut me a new one that worked great, for no extra charge.
Insider tip: look on the bottom shelf of one of the middle produce shelves, and you'll see wrapped up packs of clearance produce for $1 each. It's a bit of a gamble, but I've gotten great oranges, pears, and even avocados this way!