Definitely go to the Aquatic Design Centre! It's my LFS since i only live 10 min from it, it's a super place.
King's Aquatics & Reptiles in Camden has some nice fish too, i live opposite it virtually, but the aquatics section is closed for refurbishment at the moment, although it might have opened again by now, i need to pop in again. I went in two weeks ago and they said it'd be back up and running in two weeks...