I haven't found better quality fresh fish in London.
Moxons - http://www.moxonsfreshfish.com/ - sources its fish daily from the south coast (Looe for instance), Billingsgate market, and Loch Duart in Scotland (salmon).
Everything on ice in the shop is fresh and fantastic quality - almost every whole fish looks, feels and smells like it's still alive. The eyes are alive and clean, the smell is of the sea but not 'fishy', and the flesh has a nice bounce.
There's a good, seasonal variety - with Colchester oysters, Devon crabs, Cornish bream and mackerel, clams, razor clams, sole - I could go on. There are of course the more unsustainable varieties such as tuna and cod, and while the shop claims that all of it is sustainable, I have doubts and have personally stopped eating both those species. With so much local sustainable types such as mackerel, farmed bream and pollock, who needs to eat the under pressure species?
I find the staff here are friendly and helpful - always able to find you something for a recipe you have in mind, or vice versa, suggesting recipes for fish they have on the day.
Prices vary depending on the market price - which is often influenced by what's fashionable in London restaurants - but you can get some incredible fish for good prices if you know what you're buying. Mackerel, bream and razor clams for instance all offer superb value.
On weekends, typically the place is busy with long queues - but it's worth the wait. Get there early for the best choice, or late for the best bargains. My girlfriend and I were given a sack of mussels - enough for 10 - for half the price they should have been.
If you like good quality fish, then this is the place for you. read more