Not a breeder but part of my job is to license breeders- which is now the law no matter how many litter you produce.
Each breeder must apply for a license. They have to follow some very specific guidance on accommodation, hygiene, safety, socialisation, paperwork etc
For example someone must be in the premises at all time, puppies must be weighed at birth and regular intervals after that. Strict conditions on shelling facilities, temperature, size of living conditions. There must be a separate exercise area that is not where the dog sleeps. They must be checked and interacted with 4 times a day.
You must have a fire safety plan, you must have an infectious outbreak plan
Etc etc
Bitches must not be bred from before 12months( ideally 18months) or after 8 years old.
There must be at least 12 months from weaning to mating again (to prevent back to back litters)
Puppies must be shown with mum to prospective buyers
You must have a puppy contract.
Each specific license will say how many dogs you can have in the premises.
Most places have mostly bitches with a few dogs.
You can make a living out of it, and actually it’s not really something you can do after work etc
You would need good stock, a 5* license and to build up a good reputation.
You need to declare all income to HMRC.
You are inspected when you first set up, including a vet check of all the dogs on the premises.
You then have twice yearly inspections, including one unannounced.
Every single bit of paperwork will be inspected.