
DEFRA's rules are that you need a license if you breed more than 4 litters a year. However certain councils have their own rules, usually that you need a license if you own 3 entire bitches and breed from them, regardless of number of litters.
Most show breeders are not licensed and although obviously it isn't something true of everyone, very often somebody having a breeding license makes them look suspect to other breeders -not many good breeders breed as much as to need one. There are certainly exceptions but your average good, responsible breeder don't breed as much as this so a license may act as a red flag to buyers -are they in fact puppy farmers?