I live in bethnal green and go to wholesale aquatics as my LFS. They are generally friendly and can answer my questions. Best of all they are walking distance from me.
I have been to the fish shop on great Portland st and its expensive! they do have everything you could want though so if im in a pinch i will go there.
There is another in my neighborhood called J&L aquatics (or something like that) on old ford road and i wouldn't send my worst enemy to it.