The London Aquarium is good, though expensive.
The main problem with it is that as far as information goes, it sucks majorly. Apart from a few small labels telling you what the fishes on display are, there's nothing on ecology, behaviour, or physology. From an educational perspective, the place gets 1 out of 10. I think they rely more on having "explainers" lead tours and such, which is fine, but mostly you see people looking at fishes swimming about but not actually learning anything. As someone into fish, I find that hugely frustrating.
On the plus side, the brackish water display is fun, and the smaller coldwater tanks especially are very inspirational. Be sure and bring your camera.
Cheers,
Neale