I'm not sure where Margaret Thatcher did her food shop so that she could so confidently proclaim that we "(are) all middle class now", but I'm pretty sure I've not been there. Whether or not you subscribe to traditional theories of the British class system or feel that the modern world has led to an erosion or a convergance, it's hard to deny that social strata are still alive and well in supermarkets.
I'm not going to claim that people rigorously stick to their class in terms of supermarket, or that there is aspirational upward mobility - the majority of food shopping choice comes down to geographical convenience, economic circumstance and product range, after all - but there is still a very real sense of class lining our supermarket shelves.
Tesco and Asda. They're your working class shops. The food is cheap and cheerful. Nothing too peacocky (and certainly no frozen oven-ready peacocks) or anything with a flamboyant garnish that would end up with you getting nutted in the car park at The Rat and Trumpet later on. Offers on powerfully strong lager are plentiful and very generous, so as to keep the patrons in enough of a fug of drunkeness that they never achieve class consciousness. Clientele include careful pensioners, 15-year old mothers of two and children called Jayden and Candis who climb the shelves in the Kosher food aisle and throw Matzos at everyone, whilst shrieking like apes.
Morrisons and the Cooperative, that's your lower-middle class supermarket. They have low enough prices because you're probably saving up for a conservatory that will look out of place and probably won't ever be finished or the roof will leak. But it's also the sort of place where you feel a step up from Asda. Yes, you can still get 24 Stella for a tenspot, but you can also buy chorizo and mascarpone for definite.
Sainsbury's, there's your middle class supermarket. The biscuits come in boxes and are delicious but ironically the clientele are more likely to set your teeth on edge than those in Morrisons or the Co-op. The people who shop there have one eye on improving themselves and for the most part make their children behave or tie them up to a lamppost outside. In Sainsbury's people are largely happy with their lot. They (badly and staggeringly thoughtlessly) park their Lexus across two parent and child spots and then let little Tarquin and Poppy run around, pointing at the poor and eat grapes that haven't been paid for yet and NEVER WILL.
Waitrose is the upper-middle class supermarket. The food is excellent. You even know for sure that the fruit is going to be good, because they source it right. It's the sort of shop where you can get artisan cheese and venison and bulgar wheat as impulse buys by the checkouts. But the people in there are just the worst kind of hempy, superior, Margot Leadbetter titwitches. The majority of the women in there take a footstool to facilitate looking down their nose at you, and are likely to assume anyone not wearing a mink stole is on the staff. Their husbands, still virtually moribund from a solid decade of marijuana and acid in the 1960s and 70s, made their fortune by being Status Quo's road crew but now can barely speak and shuffle behind obediently. Even the people on the checkout think you're an escaped convict unless you've got a micro pig in your bag for life. They don't sell large sizes of anything because they wouldn't go with the Smeg fridge and no-one who has ever shopped there realises that a chicken even has wings. Upside: no-one who shops regularly in Waitrose has children, even the married couples. Some of whom may even have sold theirs.
Marks and Spencers is your upper class supermarket. You can't get proper brands, you have to get their own which, in true upper class style are more expensive and worse. Most of the fruit in there is pre-prepared so as to save the houseboy a job. But the clientele are nicer than the Waitrose lot, as are the clientele in any shop where the fact they would rather that you'd die is largely absent from their facial expressions.
There are other shops but I'm either too posh or too common to have been in them. The end.