Mille Fleur is just the coloring. Belgian Bearded d'Uccle bantams come in various colors/patterns, one of which is Mille Fleur. They are a pure bantam breed, which means there is no standard size of that breed. Cochins, on the other hand, are not a pure bantam. They also come in a standard size. They are not from Belgium, they are from China. But they can be Mille Fleur colored. SO, there IS a difference between a Mille Fleur and a Cochin if what you mean is Mille Fleur d'Uccle BREED.
I also found on Wikipedia that Pekin bantams are a true bantam--no standard counterpart. They are sometimes erroneously called Cochin bantams (probably because they look like Cochins and they are from China). They aren't Cochins, but there IS such a thing as Cochin bantams, with a standard size counterpart.
Dutch Booted bantams are the same as Belgian d'Uccles except they don't have the beard. d'Uccles look like they have a fluffy face because they have long feathers on their cheeks and chin. It's super cute (we have d'Uccles).
Chickens are broken down into 2 main Categories, those being Large Fowl (often mistakenly referred to as Standards), and Bantam.
So, Bantam is not a breed, it's a Category of all small breeds, most are miniatures, about 1/4 the size of their Large Fowl counterparts, but others are "True Bantams", such as Silkies, which have no Large Fowl counterpart. Other True Bantams breeds are: Booted, Belgian Bearded D'Uccles, Belgian Bearded D'Anvers, Rosecombs and Seramas. In other words they are found only as Bantams. All breeds, whether Large Fowl or Bantam have specific characteristics, or "Standards" as written in the Standard Manuals, size, weight, body shape are all specific to their breed. Colour and Patterns are specific to Variety.
The Classes of Large Fowl Chickens as described in the APA Standard of Perfection are: American, Asiatics, Continental, English, Games and AOSB (All Other Standard Breeds).
The Classes of Bantam Chickens as described in both the APA Standard of Perfection and the ABA Bantam Standard are: Feather Legged, Games, Single Comb Clean Legged, Rose Comb Clean Legged, and All Other Comb Clean Legged.