"I understand that you can be a size 16 and healthy! But it is more unusual.
Having a high percentage body fat ratio is without doubt less healthy than having a low one."
I don't think it's that unusual to be size 16 and have a reasonable body fat ratio you know - I'm size 16-18, 5'5", and body fat 18-20% which AFAIK is fairly healthy. Looking around at women I see at the gym, that doesn't seem a particularly unusual body shape amongst moderately athletic women in their 30s & 40s, which I s'pose is the lower end of the Debenhams age range.
I looked at the mannequin on the Daily Mail article, which seems to have the most pictures. What struck me was that the dress (polka dot shift with a belt) they showed on the mannequin looked absolutely terrible - whereas the size 16 mannequin in lingerie looked great, because the shapes were flattering.
So maybe it's designers not shops who really need the size 16 (and other sizes of) mannequins if they want to design clothes that flatter real women of all sizes.
Although perhaps (being cynical) that you are more likely to buy more clothes if you have wardrobe of clothes that don't quite fit or flatter.
Though personally, I'm more likely to go on a spending spree if I'm on a roll of enjoying clothes rather than feeling dissatisfied with my wardrobe.