I'm not in UK so can't advise about local business permits and requirements; but can give you some sound advice . Businesses need a written plan, a large initial investment, and a market study in order to start right. Businesses involved with live creatures also need an owner or trusted employee that is pretty much an expert in the animals involved. That knowledge extends to not only keeping the animals healthy; but of breeding and raising the eggs into sellable froglets. You also need to produce huge amounts of food for babies and connections to be able to move large amounts of froglets.

There are at forum members that breed their Darts and adopt them out to friends or forum members. Imagine there are others that sell excess locally to pet shops; but don't expect to make lots of $ that way. I used to trade fish fry (sometimes with the parents) to local shops and would get 25% of their value as credit at store when lucky . I know of a few fish lovers that have started a breeding business in their basement or a garage and never recouped the initial investment of several thousand dollars. Maintenance costs and marketing the product are probably the biggest long term challenges you will face; so might as well plan for them now before you go into debt and break the bank .