Through personal use I can highly recommend Wildwoods through "tropical fish 2 your door," the African catfish I received were excellently packed and came with a laminated A4 paper giving details of how the fish were kept by them including foodstuffs. A minor negative was that the Synodontis notata turned out to be a Synodontis congica and the Microsynodontis batesii appear to be polli (but this one is still a bit up in the air, Microsynodontis are not well documented), but these are not common fish (so more likely for wholesaler ID errors given to shop) and the fact they happened to be different species did not bother me in the slightest.
Other fishkeepers I talk to outside the forum speak well of Pier Aquatics (who recently joined the courier bandwagon, this place is almost the "fish Mecca" of the UK); EV Aquatics and Rare Aquatics.