Okay, so first of all, this isn't open 24 hours; least of all on a Sunday, when it closes at 5PM along with every other grocery store in the area. That being said, even during the lockdowns, it is open fairly late and is really only one of two stores on this street that can be considered an actual supermarket.
Enormous though it is- by UK standards- it isn't hard to get around inside and has two floors to keep things simple and easy to navigate, as well as an underground parking garage for those who dare to drive on these streets. Food and seasonal grocery-related items are on the bottom; clothing, toiletries, cookware, toys, and everything else are on the top.
The prices here are very good for most items, and the quality ranges from, "I wouldn't trust this," to "I'll take ten!" The variety alone makes it worth the trip and the rather long checkout queues, as you're likely to be able to find everything you need in one go. Just don't expect a whole lot from the staff, as most will give you a lukewarm response, at best.
If for any reason, you don't find what you're looking for, there are two other- smaller- grocery stores just a few minutes' walk down the street. And personally, I rather like walking here. Streatham High Street has a nice homey vibe to it. read more