Now I can shop in Top Shop and all the fashionable places, that must be what you mean by "Junior" shops
Junior is a size range, not really a fashion type. Misses are the even sizes 0, 2, 4, 6, 8, etc. Juniors are 00, 1, 3, 5.
So it's how the clothes are cut more than the style. I shop at H&M and they have Misses sizes but the styles are young and hip. But it's not a Junior shop. Forever 21 is a Junior shop though because they have the odd sizes.
Basically, Misses are for hour-glass figures -- people with hips & busts and Juniors are for people with a boxier / slimmer shape.
That's funny that the cheaper clothes are cut smaller in the UK. Here in the US, if you want stuff cut smaller, you go to where the rich people shop!