Breeding and Sale of Dogs (Welfare) Act 1999
Breeders who breed four or more litters per year must be licensed by their local authority. Breeders with fewer litters must also be licensed if they are carrying out a business of breeding dogs for sale.
Licensed breeders must:
a) Not mate a bitch less than 12 months old.
b) Not whelp more than six litters from a bitch.
c) Not whelp two litters within a 12 month period from the same bitch.
d) Keep accurate records.
e) Not sell a puppy until it is at least eight weeks of age, other than to a keeper of a licensed pet shop
or Scottish rearing establishment.
This is from the KC website.. Thing is though no all breeders here are licensed.
Some are hobby breeders who just dont bother to get a license or some litters are 'acidental'
This is where it comes common (usually 6 weeks more then 5).
Over here pups are weaned at about 4 weeks and once they are eating dog food they go to new homes, I know its not right and I agree it shouldnt be common but unfortunatley it is.
EDIT: Oh and just to answer your origional question yes it is illegal but unfortunatley still happends.