I’m a very staunch fan of the Marks & Spencer crisp combo mix, in every flavour (ready salted is the obvious exception, and the Christmas maple bacon is an acquired taste); the fact that some of the shapes are bad (the wheels) and some are magnificent (the shells) only adds to my admiration. It is random reinforcement, in crisp form. Consequently, when I saw an M&S was opening in Clapham Common, south London, I was pretty excited, but I thought it was just big news for me and maybe one or two other bar-snack connoisseurs.

I had reckoned without the TikTok generation, who have lost their minds over the new biscuits. I’d also overlooked the constituency of fancy people, who favour a shop that sells 17 different varieties of apple, and want to see everything in a fridge, even things that definitely don’t need refrigeration. I hadn’t given enough thought to the people who just like to go up to a wall of premixed cocktails and stare at it, but this turned out to be every single mum from the kids’ primary school, me included, so it was like a reunion. I don’t really associate M&S with health food, but the yoga-mat brigade stalked every aisle in pairs; it looked like Noah’s ark for people with very strong cores. This, my friend’s husband explained, is because middle-class people don’t worry about alcohol, or sugar, they only worry about palm oil and processing.

I only saw all that three days after it opened – the first day, it was so mobbed that the queue covered the entire high street, as if the Beatles were inside.

I apologise in advance for this deep dive into the geography of south London, but you’ll need it to join me in this mystery: there’s already an M&S in Clapham Junction, which is less than a mile away, and one in Brixton, a mile and a half away. It makes absolutely no sense for a shop to generate quite so much excitement, unless we understand it less as a consumer experience and more as a kind of cathedral, a moment to come together as a community and say, finally: “We’re people who deserve the best crisps.”

• Zoe Williams is a Guardian columnist

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