I wash cashmere if it’s labelled as washable. Never had a problem with those, but I wouldn’t risk washing this as it’s labelled do not wash. As it’s from M+S who sell washable wool knitwear ….I don’t think you can assume that they just slap a Do Not Wash label onto all their cashmere.
You could try wetting a tiny area on the back and see what happens? But if it were mine, I wouldn’t risk it, keep it for best, and it should only need dry cleaning rarely. Btw if you’re not wearing it that often, moths love cashmere and the more so if you’ve worn it, but you can keep it safe by storing it somewhere light where it’ll be disturbed regularly.