We've got a perfectly adequate system - 12, 14, 16, etc - so why use S, M, L, etc? Especially when what the letters represent can vary from shop to shop.
And even more especially when L = size 16.
16 is hardly large! Isn't it the average size in the UK?
Oh, and, go on then, really rub it in by labeling sizes 18, 20, 22 etc XL, XXL, XXXL etc!
Why can't shops just stick to the numerical system?