go to
www.tropicalfishfinder.co.uk then to the tropical fish to your door (TF2YD) part. there is wildwoods and wholesale tropicals.
i have ordered from wildwoods and the fish i recieved were of excellent quality. my GT grew to 6" or so before i gave him away due to possible aggression. he was a beauty. highly recommended. i have never bought from one called dartford aquatics but when they were on TF2YD they had very poor ratings, often 1 or 2 stars out of 5, saying about leaking bags, dead fish, dying fish due to leaking bag destroying the heat pack etc etc. they arent on it now but you can still buy online from them.
there is also atlantis aquatics but they seem pricey to me.
also
www.aquaticlifedirect.co.uk, i bought from these when they traded under a different name and was not impressed by the way they packaged the fish, clear plastic with metal ties on the ends to make a bag.
www.cheapfish.co.uk, but if you look at his stock list some are a joke. he has festaes under 2 names and different prices and claims that a luminescant purple/yellow parrot fish is not dyed but natural.
www.directtropicals.co.uk, never bought from him before so i am unsure how his stock is. he does do a starter pack though made of various fish to start a small community.
i also hear good things about monty at Tri-mar. and they have an extensive list of livestock.