I am looking for recommendations for a good (live) fish shop. We're getting a small aquarium for our young daughter's birthday and we're looking for a local shop to sell us some friendly, colourful low-maintenance fish.
There is definitely nowhere in SE1. I'd search for fish forums and find a supplier/breeder from there. I'm not a fish keeper but if there is such a thing as hereditary illness in fish then a good breeder will not supply a shop. A good breeder of any pet needs feedback about all the animals they produce.
Fin King have got quite a reputation amongst reptile breeders. Like I pointed out, it's only possible to breed ethically if you can get feedback from the people that take on your animals. The odd healthy animal means nothing statistically.
I thoroughly recommend the Aquatic Design Co (107 Gt Portland St). I personally think they are cheaper and better than Finking. They sell everything you could want. The shop is on 2 levels with downstairs being all sea water fish (which are beautiful) cold and tropical fish are all on the ground floor. I'm sure you won't be disappointed.