The advert is stupid, like most of them, very irritating.
I used to buy a few bits from M & S, now I only go in there for food. I am 5'2", size 12 and saw a nice dress in there the other day, I tried it up against myself (for a laugh) and there was at least 4" of it dragging on the floor!
They used to make some items in 'petite' fittings but they seem to have stopped that now. I know younger women are taller but there are still plenty of us shorties around!
I have been trying to buy cropped leggings too, I like my tunics but the best place for cropped leggings in nice colours now seems to be my favourite shop, Seasalt.