GrandmaKT, that was my reaction when I heard this problem. It would never occur to me to even try to do a 'proper' food shop in M&S.
I started shopping in Waitrose about 30 years ago when all the main supermarkets around methen began expanding even more and it was taking me longer and longer to do the usual weekly shop for a family of 4.
I then realised there was shopping precinct with a Waitrose locally, the Waitrose was much smaller ten the other big ones but big enough for me to get all I wanted - and it was 15 minutes quicker to shop there. There were also banks, chemists, newsagents, hairdressers and all the other shops one needs in the same location so that I could do all the shopping etc I needed in one lunchtime and be back in work on time.
When we moved to our current house the situation was replicated. All three small towns near us have a Waitrose in the town centre, where one weekly visit enables me to go to the supermarket and do almost any other shopping I want to do in one visit. The alternative is a couple of out of town huge supermarkets, which remain too big to shop in efficiently and require a separate journey to town for hair cuts, bankes etc.
There is an M&S food in another local town, it opened a few months ago, but it requires a special journey as it is not a town I usually use.
The food in Waitrose is excellent and food quality and good welfare standards for animal products and for those growing tropical products is very important to me. I am sure they are not perfect, who is, but I trust them more than I would trust most of the alternative supermarkets.