they're just a shop that has no power over any one
A tad naive perhaps? A massive household-name retailer with branches all over the country? If M&S promotes and normalises the notion that some clothing is 'modest' and the rest is, by implication not modest, it is part of a drip-drip approach to moralizing over and policing women's choices.
It is sexist. Can you think of any other reason why there isn't an equivalent men's 'modest' section.
The religious or cultural rules that give rise to this notion of modest female dress are sexist. No point pretending otherwise. However, people are free to practise a religion that is a bit sexist, very sexist or horrifically sexist- totally their choice. But why should those rules about dress - rules that have no logical reason to exist outside of a patriarchal religion - have any space in a secular setting?
If M&S want to sell a range that explicitly caters for religious groups- well, ok - why not? But NAME IT. Name it properly. Don't choose a word that implies you have bought into the moral value judgements about women that we are desperately trying to eradicate.
Of course M&S can call it modest or moralwear or whatever misogynistic label they want to come up with.
They can - but, equally, we can object.
I won't 'chillax' about it, no.